Ring round for a smile (4)
I believe the answer is:
grin
'for a smile' is the definition.
('grin' can be a synonym of 'smile')
'ring round' is the wordplay.
'round' indicates anagramming the letters.
'ring' is an anagram of 'GRIN'.
(Other definitions for grin that I've seen before include "Smile - all that could be seen of the vanishing Cheshire Cat" , "Ring (anag.)" , "Beam" , "Cheesy smile?" , "Cheeky smile" .)