In support of (3)
I believe the answer is:
for
'in support of' is the definition.
(as in being for a particular cause)
This is the entire clue.
(Other definitions for for that I've seen before include "In favour of; since" , "Approving" , "On the side of" , "Favouring" , "and 16 and 20: Notwithstanding" .)