So taken in by enthusiasm (4)
I believe the answer is:
thus
'so' is the definition.
(I know that so can be written as thus)
'taken in by enthusiasm' is the wordplay.
'taken in by' indicates a hidden word.
'THUS' can be found hidden inside 'enthusiasm'.
(Other definitions for thus that I've seen before include "Therefore, hence" , "It follows" , "as indicated" , "In this manner" , "As a result" .)