Expect leading actor will appear in theatre (5)
I believe the answer is:
await
'expect' is the definition.
(synonyms)
'leading actor will appear in theatre' is the wordplay.
'leading' says to take the initial letters (the leading letter of the word).
The first letters of 'actor will appear in theatre' is 'AWAIT'.
(Other definitions for await that I've seen before include "Hid a microphone in; bothered" , "Remain in expectation for (an occurrence)" , "Look for; be in store" , "Be in expectation of (an event)" , "Be ready for" .)