Wild enthusiasm for a player, a first-class back? (5)
I believe the answer is:
mania
'wild enthusiasm' is the definition.
(I've seen this in another clue)
'player a first-class back?' is the wordplay.
'player' becomes 'man'.
'a first class' becomes 'ai' (A1 - top quality).
'back?' says the letters should be written in reverse (I've seen 'back' mean this).
'ai' written backwards gives 'ia'.
'man'+'ia'='MANIA'
'for a' is the link.
(Other definitions for mania that I've seen before include "Passion -- mood disorder" , "Frenzy" , "A (violent) mood disorder" , "Aberration" , "Excessive enthusiasm" .)