Waste doctor on old ship (5)
I believe the answer is:
dross
'waste' is the definition.
(I know that dross is a type of waste)
'doctor on old ship' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'dr'+'o'+'ss'='DROSS'
(Other definitions for dross that I've seen before include "Scum on molten metal" , "Lees" , "Scum on molten metals" , "Scum - dregs" , "Worthless or dangerous material" .)