Metal splits away on the wagon (9)
I believe the answer is:
abstinent
'on the wagon' is the definition.
(I've seen this in another clue)
'metal splits away' is the wordplay.
'metal' becomes 'tin' (tin is a kind of metal).
'splits' means one lot of letters goes inside another (inserted letters split open the word).
'away' becomes 'absent' (similar in meaning).
'tin' put inside 'absent' is 'ABSTINENT'.
(Other definitions for abstinent that I've seen before include "Restrained" , "On the wagon" , "self-restraint" , "Refraining from drinking alcohol" , "Keeping off alcohol" .)