Amateur enthusiast coaches snooker player (12)
I believe the answer is:
trainspotter
'amateur enthusiast' is the definition.
(amateur enthusiastic about trains )
'coaches snooker player' is the wordplay.
'coaches' becomes 'trains' ('train' can be a synonym of 'coach').
'snooker player' becomes 'potter' (someone who pots snooker balls).
'trains'+'potter'='TRAINSPOTTER'
(Other definitions for trainspotter that I've seen before include "anorak" , "Gricer" , "Boring person" , "Nerd" , "One interested in engines" .)