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

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "subserve":
   act for, advance, assist, be found wanting, be inferior,
   be instrumental, come short, conduce to, contribute to, decide,
   determine, facilitate, fail, fall short, follow, forward,
   go between, have nothing on, influence, lead to, mediate,
   minister to, not approach, not come near, not compare, not hack it,
   not make it, not measure up, play second fiddle, promote,
   rank under, redound to, serve, turn the scale, want