Small convict found mining waste (4)
I believe the answer is:
slag
'mining waste' is the definition.
(I've seen this before)
'small convict' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'convict' becomes 'lag' (I've seen this before).
's'+'lag'='SLAG'
'found' is the link.
(Other definitions for slag that I've seen before include "Mound of waste matter from mine" , "Perfect" , "Waste from ore refining" , "Coal mining waste" , "Solid waste matter as from mine or smelter" .)