Tow-headed girl, a beautiful swimmer (4)
I believe the answer is:
tuna
'a beautiful swimmer' is the definition.
'tuna' can be an answer for 'swimmer' (tuna is a type of fish). I'm not certain of the 'a beautiful' bit.
'tow-headed girl' is the wordplay.
'tow-headed' becomes 't' (this could be a standard abbreviation which I don't know about).
'girl' becomes 'Una' (girl's name).
't'+'una'='TUNA'
Can you help me to learn more?
(Other definitions for tuna that I've seen before include "Type of fish" , "(Meat of) large fish" , "Large tasty fish, fond of a nut" , "Large food fish" , "Marine food for your strange aunt" .)