Firm sort of wave in middle of kiss (8)
I believe the answer is:
business
'firm' is the definition.
(synonyms)
'sort of wave in middle of kiss' is the wordplay.
'sort of wave' becomes 'sine'.
'in middle of' means one lot of letters goes inside another.
'kiss' becomes 'buss' (synonyms).
'sine' placed inside 'buss' is 'BUSINESS'.
(Other definitions for business that I've seen before include "Concern; commerce" , "Affair - matters to be attended to" , "Work - shop" , "Profession - trade - matter" , "Baby" .)