During the interval is to oppose (6)
I believe the answer is:
resist
'oppose' is the definition.
(resisting is a kind of opposing)
'during the interval is' is the wordplay.
'during' indicates putting letters inside.
'the interval' becomes 'rest' (rest is a kind of interval).
'rest' enclosing 'is' is 'RESIST'.
'to' is the link.
(Other definitions for resist that I've seen before include "Check; withstand" , "Don't give in" , "Put up opposition, defy" , "Refuse to comply" , "Use force to prevent something from happening to sister" .)