To stand up to rest is unusual (6)
I believe the answer is:
resist
'to stand up to' is the definition.
(thesaurus)
'rest is unusual' is the wordplay.
'unusual' is an anagram indicator.
'rest'+'is'='restis'
'restis' with letters rearranged gives 'RESIST'.
(Other definitions for resist that I've seen before include "Combat" , "Endure" , "Withstand a force" , "Put up fight, oppose forcefully" , "Offer opposition to" .)