Consideration in favour of incorporating legislation (6)
I believe the answer is:
factor
'consideration' is the definition.
(I've seen this in another clue)
'in favour of incorporating legislation' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'incorporating' means one lot of letters goes inside another.
'legislation' becomes 'act' (I've seen this before).
'for' enclosing 'act' is 'FACTOR'.
(Other definitions for factor that I've seen before include "One circumstance that contributes to a result" , "Contributory influence; business agent" , "Contributing element" , "something that may affect result" , "Circumstance that contributes toward a result - arithmetically?" .)