So enthusiastic? Not entirely! (4)
I believe the answer is:
thus
'so' is the definition.
(I know that so can be written as thus)
'enthusiastic? not entirely' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'THUS' can be found hidden inside 'enthusiastic'.
(Other definitions for thus that I've seen before include "As a result" , "Like so" , "as indicated" , "In this way" , "Consequently" .)