Close up of a swimmer (4)
I believe the answer is:
seal
'close up' is the definition.
(I've seen this before)
'a swimmer' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
The remaining letters 'sel' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for seal that I've seen before include "Amphibious sea animal or kind of stamp" , "marine creature" , "Close tightly - mammal" , "Ratify" , "Sea-dweller can produce a wax impression" .)