In a terrible struggle, it's for fear (4)
I believe the answer is:
lest
'for fear' is the definition.
(I've seen this in another clue)
'in a terrible struggle it's' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LEST' is hidden in the letters of 'terrible struggle ts'.
(Other definitions for lest that I've seen before include "For fear that (something happens)" , "For fear that (something should happen)" , "In order not to, say forget?" , "In case" , "In order not to, to avoid the risk that" .)