Opposing sister, upsetting worker (9)
I believe the answer is:
resistant
'opposing' is the definition.
(I've seen this in another clue)
'sister upsetting worker' is the wordplay.
'upsetting' indicates anagramming the letters.
'worker' becomes 'ant' (worker is a type of ant).
'sister' is an anagram of 'resist'.
'resist'+'ant'='RESISTANT'
(Other definitions for resistant that I've seen before include "unlikely to be damaged?" , "Able to withstand adverse conditions" , "Withstanding adverse conditions" , "(Of germs) withstanding treatment" , "repellent" .)