Female visiting good, fairly elderly swimmer (8)
I believe the answer is:
goldfish
'swimmer' is the definition.
(goldfish swim)
'female visiting good fairly elderly' is the wordplay.
'female' becomes 'f' (abbreviation).
'visiting' is an insertion indicator.
'good' becomes 'g' (abbreviation).
'fairly elderly' becomes 'oldish' (somewhat old).
'g'+'oldish'='goldish'
'f' put inside 'goldish' is 'GOLDFISH'.
(Other definitions for goldfish that I've seen before include "Ornamental reddish Chinese carp (8)" , "Domestic pet with an approximate 11-second memory" , "Pond or aquarium dwellers" , "Bowl or pond dweller" , "Place lacking privacy" .)