Possibly corny item to advertise swimmer's aid (7)
I believe the answer is:
earplug
'swimmer's aid' is the definition.
(a swimmer might wear earplugs)
'possibly corny item to advertise' is the wordplay.
'possibly corny item' becomes 'ear' (ear of corn).
'to advertise' becomes 'plug' (plugging is a kind of advertizing).
'ear'+'plug'='EARPLUG'
(Other definitions for earplug that I've seen before include "Noise reducer" , "silencer" , "Noise protector" , "It blocks out sound" , "Device to keep out noise" .)