Doctor on old ship? Rubbish (5)
I believe the answer is:
dross
'rubbish' is the definition.
(I've seen this before)
'doctor on old ship?' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'on' says to put letters next to each other (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 metals" , "Slag!" , "Impurity" , "Lees" , "Waste coal - rubbish" .)