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 other clues)
'in a terrible struggle it's' is the wordplay.
'in' says the answer is hidden in the clue.
'LEST' is hidden within '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" .)