The Collaborative International Dictionary of English v.0.48:

Subagent \Sub*a"gent\, n. (Law)
   A person employed by an agent to transact the whole, or a
   part, of the business intrusted to the latter. --Bouvier.
   Chitty.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

SUB-AGENT. A person appointed by an agent to perform some duty, or the whole
of the business relating to his agency.
     2. Sub-agents may be considered in two points of view. 1. With regard
to their rights and duties or obligations, towards their immediate
employers. 2. As to their rights and obligations towards their superior or
real principals.
     3.-1. A sub-agent is generally invested with the same rights, and
incurs the same liabilities in regard to his immediate employers, as if he
were the sole and real principal. To this general rule there are some
exceptions for example, where by the general usage of trade or the agreement
of the parties, sub-agents are ordinarily or necessarily employed, to
accomplish the ends of the agency, there, if the agency is avowed, and the
credit is exclusively given to the principal, the intermediate agent may be
entirely exempted from all liability to the sub-agent. The agent, however,
will be liable to the sub-agent, unless such exclusive credit has been
given, although the real principal or superior may also be liable. Story on
Ag. Sec. 386; Paley on Ag. by Lloyd, 49. When the agent employs a sub-agent
to do the whole, or any part of the business of the agency, without the
knowledge or consent of his principal, either express or implied, the latter
will only be entitled to recover from his immediate employer, and his sole
responsibility is also to him. In this case the superior or real principal
is not responsible to the sub-agent, because there is no privity between
them. Story on Ag. Sec. 13, 14, 15, 217, 387.
     4.-2. Where by an express or implied agreement of the parties, or by
the usages of trade, a sub-agent is to be employed, a privity exists between
the principal and the sub-agent, and the latter may justly maintain his
claim for compensation, both against the principal and his immediate
employer, unless exclusive credit is given to one of them; and, in that
case, his remedy is limited to that party. 1 Liv. on Ag. 64; 6 Taunt. R.
147.