Different in feel, seriously (4)
I believe the answer is:
else
'different' is the definition.
(synonyms)
'in feel seriously' is the wordplay.
'in' indicates a hidden word.
'ELSE' is hidden within 'feel seriously'.
(Other definitions for else that I've seen before include "In addition, besides" , "Adverb meaning if not, otherwise" , "In addition; different" , "Eels (anag)" , "Besides, otherwise" .)