Cook right on the oven (5)
I believe the answer is:
roast
'cook' is the definition.
(I know that roast is a more specific form of cook)
'right on the oven' is the wordplay.
'right' becomes 'r' (common abbreviation).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'the oven' becomes 'oast' (type of kiln).
'r'+'oast'='ROAST'
(Other definitions for roast that I've seen before include "Reprimand severely" , "Feel hot" , "Sunday tradition" , "Heat strongly - criticise excessively" , "Honey-based drink" .)