Deterioration in sport (4)
I believe the answer is:
wear
This clue is a double definition.
'deterioration' is the first definition.
(wear is a kind of deterioration)
'sport' is the second definition.
(thesaurus)
'in' acts as a link.
(Other definitions for wear that I've seen before include "Dress in; erode" , "Tackle" , "Northern river - sport" , "Tolerate" , "Have on - deteriorate from constant use" .)