'Unenthusiastic' is large word (5)
I believe the answer is:
loath
'unenthusiastic' is the definition.
(thesaurus)
'large word' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'word' becomes 'oath' (I've seen this in another clue).
'l'+'oath'='LOATH'
'is' is the link.
(Other definitions for loath that I've seen before include "Unwilling" , "Reluctant or averse" , "Dislike intensely" , "Hateful" .)