The Collaborative International Dictionary of English v.0.48:

Underplay \Un`der*play"\, v. i.
   1. To play in a subordinate, or in an inferior manner; to
      underact a part.
      [1913 Webster]

   2. (Card Playing) To play a low card when holding a high one,
      in the hope of a future advantage.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underplay \Un"der*play`\, n. (Card Playing)
   The act of underplaying.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underplay
    v 1: act (a role) with great restraint [syn: underact,
         underplay] [ant: ham, ham it up, overact,
         overplay]
    2: play a card lower than (a held high card)

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "underplay":
   abate, allay, alleviate, assuage, attemper, bank the fire, bedwarf,
   belittle, blunt, chasten, constrain, control, damp, dampen,
   de-emphasize, deaden, deprecate, depreciate, detract from,
   diminish, downplay, dull, dwarf, extenuate, keep within bounds,
   lay, lenify, lessen, lighten, make light of, make little of,
   make nothing of, minify, minimize, mitigate, moderate, modulate,
   obtund, palliate, play down, reduce, reduce the temperature,
   restrain, set at naught, set little by, slacken, slow down,
   smother, sober, sober down, soften, stifle, subdue, suppress, tame,
   temper, think little of, think nothing of, tone down, tune down,
   weaken