To love immoderately (3)
I believe the answer is:
too
'immoderately' is the definition.
(thesaurus)
'to love' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'to'+'o'='TOO'
(Other definitions for too that I've seen before include "Likewise" , "Furthermore" , "As well, also" , "As well; in addition" , "Also; overly" .)