Swallow in trepidation as promotion rejected (4)
I believe the answer is:
gulp
'swallow in trepidation' is the definition.
(gulp can mean to swallow from fear or apprehension)
'promotion rejected' is the wordplay.
'promotion' becomes 'plug' (plug is a kind of promotion).
'rejected' shows that the letters should be reversed in order.
'plug' backwards is 'GULP'.
'as' is the link.
(Other definitions for gulp that I've seen before include "Swallow audibly" , "Swallow hurriedly" , "Audible swallow" , "Noise made when swallowing too quickly" , "Swallow noisily" .)