Stand up against support one's brought in (6)
I believe the answer is:
resist
'stand up against' is the definition.
(thesaurus)
'support one's brought in' is the wordplay.
'support' becomes 'rest' (rest is a kind of support).
'one's' becomes 'is' (Roman numeral).
'brought in' indicates putting letters inside.
'rest' going around 'is' is 'RESIST'.
(Other definitions for resist that I've seen before include "Check; withstand" , "Withstand or defy" , "Defy for sister" , "Combat" , "Stand up to, fight against" .)