Help actor in play, in small part (6)
I believe the answer is:
prompt
'help actor' is the definition.
(remind an actor of their lines)
'play in small part' is the wordplay.
'play' becomes 'romp' (synonyms).
'in' means one lot of letters goes inside another.
'small part' becomes 'pt' (abbreviation; 'small' suggests a short form).
'romp' placed inside 'pt' is 'PROMPT'.
'in' acts as a link.
(Other definitions for prompt that I've seen before include "Whisper line to forgetful actor" , "Incite - punctual" , "Reminder" , "Help forgetful actor with lines" , "Quick - cue" .)