Ominous feeling now chaps are trapped in it (12)
I believe the answer is:
presentiment
'ominous feeling' is the definition.
(a feeling of impending disaster)
'now chaps are trapped in it' is the wordplay.
'now' becomes 'present' (the present time).
'chaps' becomes 'men' (chap means a man).
'are trapped in' is an insertion indicator.
'men' going within 'it' is 'iment'.
'present'+'iment'='PRESENTIMENT'
(Other definitions for presentiment that I've seen before include "bad feeling" , "An intuitive feeling" , "Feeling of apprehension" , "Intuitive foreboding" .)