One resisting work has a problem (7)
I believe the answer is:
Here is my best explanation:
'one resisting' is the definition.
'work has a problem' is the subsidiary indication.
'work' becomes 'op' (common abbreviation for 'opus').
'has' means one lot of letters go next to another.
'a problem' becomes 'poser' (poser is a kind of problem).
(Other definitions for opposer that I've seen before include "One against something", "The one against the motion", "Adversary", "Propose to challenger", "Second to speak", "One resisting", "Person against the motion", "One offering resistance", "One who takes on" and "Contender".)