Young swimmer in wild revel (5)
I believe the answer is:
elver
'young swimmer' is the definition.
(I've seen this before)
'wild revel' is the wordplay.
'wild' indicates anagramming the letters.
'revel' anagrammed gives 'ELVER'.
'in' acts as a link.
(Other definitions for elver that I've seen before include "New slipper" , "Immature eel" , "Little fish" , "One may see one upriver" , "Young swimmer" .)