Is interrupting relaxation so oppose (6)
I believe the answer is:
resist
'oppose' is the definition.
(resisting is a kind of opposing)
'is interrupting relaxation' is the wordplay.
'interrupting' means one lot of letters goes inside another.
'relaxation' becomes 'rest' (synonyms).
'is' placed inside 'rest' is 'RESIST'.
'so' acts as a link.
(Other definitions for resist that I've seen before include "Forgo" , "Put up fight, oppose forcefully" , "Withstand or defy" , "Withstand a force" , "Withstand, oppose" .)