Lad seen swimming with fish (4,3)
I believe the answer is:
sand eel
'fish' is the definition.
(sand eel is a kind of fish)
'lad seen swimming' is the wordplay.
'swimming' indicates anagramming the letters.
'lad'+'seen'='ladseen'
'ladseen' is an anagram of 'SAND-EEL'.
'with' acts as a link.
(Other definitions for sand eel that I've seen before include "Very small fish that burrows into beaches" , "Shallow-water fish" , "swimmer" .)