Tread boards in leading parts at Criterion Theatre (3)
I believe the answer is:
act
'tread boards' is the definition.
(to 'tread the boards' is to act on stage)
'in leading parts at criterion theatre' is the wordplay.
'in leading parts' says to take the initial letters (letters which lead the words).
The first letters of 'at criterion theatre' is 'ACT'.
(Other definitions for act that I've seen before include "Major section of a play" , "Behave like thespian" , "turn" , "work" , "Perform theatrically" .)