Foreboding -- tense people gripped by it (12)
I believe the answer is:
presentiment
'foreboding' is the definition.
('presentiment' can be a synonym of 'foreboding')
'tense people gripped by it' is the wordplay.
'tense' becomes 'present' (present is a kind of tense**).
'people' becomes 'men' (man is a kind of person).
'gripped by' is an insertion indicator.
'men' placed inside 'it' is 'iment'.
'present'+'iment'='PRESENTIMENT'
(Other definitions for presentiment that I've seen before include "Feeling of apprehension" , "Intuitive foreboding" , "An intuitive feeling" , "bad feeling" .)