First sign of horse and trap reversing is imminent (4)
I believe the answer is:
nigh
'imminent' is the definition.
(I've seen this in another clue)
'first sign of horse and trap reversing' is the wordplay.
'first sign of' indicates taking the first letters.
'and' says to put letters next to each other.
'trap' becomes 'gin' (gin trap is a kind of animal trap).
'reversing' is a reversal indicator.
The first letter of 'horse' is 'h'.
'gin' reversed gives 'nig'.
'h' put after 'nig' is 'NIGH'.
'is' is the link.
(Other definitions for nigh that I've seen before include "Nearby" , "See 23" , "'Almost, near (4)'" , "Adjacent" , "old touch" .)