Resistance to wear head-covering seen in swimmer (4)
I believe the answer is:
carp
'swimmer' is the definition.
(carp is a kind of fish)
'resistance to wear head-covering' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'to wear' is an insertion indicator (to wear something is to be covered up by it).
'head covering' becomes 'cap' (I've seen this in another clue).
'r' inserted inside 'cap' is 'CARP'.
'seen in' acts as a link.
(Other definitions for carp that I've seen before include "Complain continually" , "Fish; nag pettily" , "Nagging fish?" , "Find fault with a freshwater fish" , "Fish - complain" .)