Dance right, swimmer (4)
I believe the answer is:
reel
'dance' is the definition.
(I know that reel is a type of square dance)
'right swimmer' is the wordplay.
'right' becomes 'r' (common abbreviation).
'swimmer' becomes 'eel' (eel is a type of fish).
'r'+'eel'='REEL'
(Other definitions for reel that I've seen before include "Rock" , "Stagger around in the dance" , "Wind" , "Dance; winding cylinder" , "Air" .)