Act for theatre to feel bitter about (9)
I believe the answer is:
represent
'act for' is the definition.
(I've seen this before)
'theatre to feel bitter about' is the wordplay.
'theatre' becomes 'rep' (repertory theatre).
'to feel bitter about' becomes 'resent' (synonyms).
'rep'+'resent'='REPRESENT'
(Other definitions for represent that I've seen before include "Stand for or symbolise" , "show" , "Illustrate" , "Act for" , "Portray" .)