Adequate swimmer gulping oxygen? (7)
I believe the answer is:
fairish
'adequate' is the definition.
(thesaurus)
'swimmer gulping oxygen?' is the wordplay.
'swimmer' becomes 'fish' (fish swim).
'gulping' is an insertion indicator (gulp can mean to swallow or consume).
'oxygen?' becomes 'air' (air contains oxygen).
'fish' enclosing 'air' is 'FAIRISH'.
(Other definitions for fairish that I've seen before include "quite OK?" , "Quite reasonable" , "Relatively light" , "rather blonde" , "Average" .)