To love unreasonably (3)
I believe the answer is:
too
'unreasonably' 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 "As well; in addition" , "Additionally" , "On top - also" , "As well, also" , "Moreover, in addition" .)