In favour of raising the roof, nothing less (3)
I believe the answer is:
for
'in favour of' is the definition.
(being for a particular cause)
'raising the roof nothing less' is the wordplay.
'raising' says the letters should be written in reverse (in a down clue, letters go up).
'nothing' becomes 'O' (looks like zero - 0).
'less' suggests deleting specific letters (less can mean minus).
'roof' with 'o' removed is 'rof'.
'rof' back-to-front is 'FOR'.
(Other definitions for for that I've seen before include "Favouring" , "Pro" , "On the side of" , "and 16 and 20: Notwithstanding" , "In support of" .)