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' indicates a hidden word.
'LEST' can be found hidden inside '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" .)