Favouring endless support (3)
I believe the answer is:
pro
'favouring' is the definition.
(I know that favouring can be written as pro)
'endless support' is the wordplay.
'endless' means to remove the last letter.
'support' becomes 'prop' (prop up can mean to support).
'prop' with its final letter taken off is 'PRO'.
(Other definitions for pro that I've seen before include "Paid worker" , "In favour of; advantage" , "Of argument, say, in favour of" , "Golf expert" , "Professional; advantage" .)