It's all right to love a girl (4)
I believe the answer is:
okay
'it's all right' is the definition.
(synonyms)
'love a girl' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'a girl' becomes 'kay' ().
'o'+'kay'='OKAY'
'to' is the link.
(Other definitions for okay that I've seen before include "Permissible; fairly good" , "Leave" , "All right; sanction" , "Satisfactory; permissible" , "Passable" .)