So, you need a bit of enthusiasm (4)
I believe the answer is:
thus
'so' is the definition.
(I know that so can be written as thus)
'a bit of enthusiasm' is the wordplay.
'a bit of' says the answer is hidden in the clue.
'THUS' is hidden in the letters of 'enthusiasm'.
'you need' acts as a link.
(Other definitions for thus that I've seen before include "'So, therefore (4)'" , "Therefore, hence" , "In this way" , "Like so" , "as indicated" .)