Underground opposition? (10)
I believe the answer is:
resistance
This clue is a double definition.
'underground' is the first definition.
(synonyms)
'opposition?' is the second definition.
(opposition is a kind of resistance)
(Other definitions for resistance that I've seen before include "Organised covert opposition to an occupying power" , "defiance" , "Withstanding" , "French fighting group" , "immunity" .)