Hold out against dire sister to some extent (6)
I believe the answer is:
resist
'hold out against' is the definition.
(thesaurus)
'dire sister to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'RESIST' is hidden within 'dire sister'.
(Other definitions for resist that I've seen before include "Hold out for sister" , "turn down" , "Check; withstand" , "'Withstand, fight against (6)'" , "Sister (anag)" .)