The Collaborative International Dictionary of English v.0.48:

Subserve \Sub*serve"\, v. i.
   To be subservient or subordinate; to serve in an inferior
   capacity.
   [1913 Webster]

         Not made to rule,
         But to subserve where wisdom bears command. --Milton.
   [1913 Webster] Subservience

The Collaborative International Dictionary of English v.0.48:

Subserve \Sub*serve"\, v. t. [imp. & p. p. Subserved; p. pr. &
   vb. n. Subserving.] [L. subservire; sub under + servire to
   serve. See Serve.]
   To serve in subordination or instrumentally; to be
   subservient to; to help forward; to promote.
   [1913 Webster]

         It is a great credit to know the ways of captivating
         Nature, and making her subserve our purposes, than to
         have learned all the intrigues of policy. --Glanvill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subserve
    v 1: be helpful or useful