Horrible struggle across line (4)
I believe the answer is:
vile
'horrible' is the definition.
(I've seen this before)
'struggle across line' is the wordplay.
'struggle' becomes 'vie' (I've seen this before).
'across' indicates putting letters inside.
'line' becomes 'l'.
'vie' going around 'l' is 'VILE'.
(Other definitions for vile that I've seen before include "Very nasty, quite evil" , "Abominable" , "Live in degenerate way" , "Disgusting, quite evil" , "Cheap; shameful" .)