Some amateur theatre in retrospect is perfect (4)
I believe the answer is:
true
'is perfect' is the definition.
(thesaurus)
'some amateur theatre in retrospect' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'in retrospect' says the letters should be written in reverse (in retrospect means looking back).
'eurt' can be found hidden inside 'amateur theatre'.
'eurt' written backwards gives 'TRUE'.
(Other definitions for true that I've seen before include "Factually correct" , "In accordance with facts" , "Veritable" , "to be believed?" , "Bona fide" .)