Payment keeping train in gear, once (7)
I believe the answer is:
raiment
'gear once' is the definition.
(archaic term for clothing)
'payment keeping train' is the wordplay.
'payment' becomes 'rent' (rent is a kind of payment).
'keeping' indicates putting letters inside.
'train' becomes 'aim' ('aim' can be a synonym of 'train').
'rent' enclosing 'aim' is 'RAIMENT'.
'in' acts as a link.
(Other definitions for raiment that I've seen before include "Clothing (literary)" , "Wear" , "dress?" , "clothing for poets" , "Traditional clothing" .)