Swimmer or runner outside shop undressed (5)
I believe the answer is:
cohoe
'swimmer' is the definition.
(cohoe is a kind of fish)
'runner outside shop undressed' is the wordplay.
'runner' becomes 'Coe' (Olympic runner Seb Coe).
'outside' means one lot of letters goes inside another.
'undressed' says to take the centre (the word removes the letters which clothe it).
The centre of 'shop' is 'ho'.
'coe' placed around 'ho' is 'COHOE'.
'or' acts as a link.
(Other definitions for cohoe that I've seen before include "Swimmer" , "Fish" , "salmon" .)