Support outside is making stand firm (6)
I believe the answer is:
resist
'stand firm' is the definition.
('resist' can be a synonym of 'stand firm')
'support outside is making' is the wordplay.
'support' becomes 'rest' (rest is a kind of support**).
'outside' indicates putting letters inside.
'making' indicates anagramming the letters.
'is' with letters rearranged gives 'si'.
'rest' going around 'si' is 'RESIST'.
(Other definitions for resist that I've seen before include "Withstand or defy" , "Combat" , "Withstand, struggle against" , "Oppose, put up defence" , "turn down" .)