The Collaborative International Dictionary of English v.0.48:

Underact \Un`der*act"\, v. t.
   To perform inefficiently, as a play; to act feebly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underact
    v 1: act (a role) with great restraint [syn: underact,
         underplay] [ant: ham, ham it up, overact,
         overplay]