Debate procedure and sit (6)
I believe the answer is:
oppose
'debate' is the definition.
(I know that oppose is a more specific form of the action debate)
'procedure and sit' is the wordplay.
'procedure' becomes 'op' (short for operation).
'and' means one lot of letters go next to another.
'sit' becomes 'pose' (as in sitting for a painter).
'op'+'pose'='OPPOSE'
(Other definitions for oppose that I've seen before include "match" , "face" , "Bar" , "Counter" , "Resist, be hostile to" .)