Guy initially is all enthusiasm (5)
I believe the answer is:
mania
'enthusiasm' is the definition.
(I've seen this in another clue)
'guy initially is all' is the wordplay.
'guy' becomes 'man' (guy can informally mean a man).
'initially' says to take the initial letters.
The initial letters of 'is all' is 'ia'.
'man'+'ia'='MANIA'
(Other definitions for mania that I've seen before include "Insanity" , "Passion -- mood disorder" , "Craze; overexcitement" , "Violent madness or extreme enthusiasm" , "thing" .)