A bit stage struck for long periods (4)
I believe the answer is:
ages
'long periods' is the definition.
(I've seen this before)
'a bit stage struck for' is the wordplay.
'a bit' indicates the answer is hidden within the clue (the answer is formed by a bit of the clue).
'AGES' is hidden within 'stage struck for'.
(Other definitions for ages that I've seen before include "... of Man, according to WB Yeats" , "Donkey's years" , "Indefinitely long time" , "Awfully long time" , "Times" .)