Slight case of reserve in front of enthusiast (6)
I believe the answer is:
rebuff
'slight' is the definition.
('rebuff' can be a synonym of 'slight')
'case of reserve in front of enthusiast' is the wordplay.
'case of' means to remove the middle letters (outsides of).
'in front of' says to put letters next to each other.
'enthusiast' becomes 'buff' (I've seen this before).
'reserve' with its centre taken out is 're'.
're'+'buff'='REBUFF'
(Other definitions for rebuff that I've seen before include "Unexpected and blunt refusal" , "Turn off" , "Drive back" , "Reject in an ungrateful manner" , "Snub or contemptuous refusal" .)