Saddle decorator over chasing source of plaster in tin (9)
I believe the answer is:
caparison
'saddle decorator' is the definition.
(decorative covering on a saddle)
'over chasing source of plaster in tin' is the wordplay.
'over' becomes 'o' (cricket abbreviation**).
'chasing' is a charade indicator (letters next to each other).
'source of plaster' becomes 'paris' (plaster of Paris is a type of plaster).
'in' indicates putting letters inside.
'tin' becomes 'can' ('can' can be a synonym of 'tin').
'o' put after 'paris' is 'pariso'.
'pariso' put inside 'can' is 'CAPARISON'.
(Other definitions for caparison that I've seen before include "covering for the Shires?" , "Cover for a horse" , "sumptuous clothing" , "What a horse may have on" , "piece of finery" .)