Swimming round catching fish (7)
I believe the answer is:
reeling
'swimming' is the definition.
(I've seen this before)
'round catching fish' is the wordplay.
'round' becomes 'ring' (synonyms).
'catching' is an insertion indicator.
'fish' becomes 'eel' (eel is a type of fish).
'ring' placed around 'eel' is 'REELING'.
(Other definitions for reeling that I've seen before include "Unable properly to control one's movements" , "Feeling giddy; dancing" , "Dancing; staggering" , "Showing effect of shock" , "Staggering about in Irish dance?" .)