Stand up to relax one's parts (6)
I believe the answer is:
resist
'stand up to' is the definition.
(thesaurus)
'relax one's parts' is the wordplay.
'relax' becomes 'rest' (I've seen this before).
'one's' becomes 'is' (Roman numeral).
'parts' means one lot of letters goes inside another (in the sense of cuts or separates).
'rest' placed around 'is' is 'RESIST'.
(Other definitions for resist that I've seen before include "Put up fight, oppose forcefully" , "Put up opposition to sister" , "Take stand against, oppose" , "Withstand, oppose" , "Defy for sister" .)