Pop off inside theatre block (8)
I believe the answer is:
stoppage
'block' is the definition.
('stoppage' can be a synonym of 'block')
'pop off inside theatre' is the wordplay.
'off' indicates an anagram.
'inside' is an insertion indicator.
'theatre' becomes 'stage' (stage is a kind of theatre**).
'pop' with letters rearranged gives 'opp'.
'opp' put inside 'stage' is 'STOPPAGE'.
(Other definitions for stoppage that I've seen before include "Onstruction in pipe or interruption in game" , "Closure" , "break" , "Instance of industrial action" , "strike action" .)