Feel bad as expert swallowing bench’s conclusion (4)
I believe the answer is:
ache
'feel bad' is the definition.
(I've seen this in another clue)
'expert swallowing bench's conclusion' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'swallowing' is an insertion indicator.
'conclusion' indicates one should take the final letters.
The last letter of 'bench' is 'h'.
'ace' going around 'h' is 'ACHE'.
'as' acts as a link.
(Other definitions for ache that I've seen before include "A dull lingering pain" , "Long ? pain" , "Dull steady pain" , "Dull throbbing pain" , "Twinge" .)