Opposing trains set off (9)
I believe the answer is:
resistant
'opposing' is the definition.
(I've seen this in another clue)
'trains set off' is the wordplay.
'off' is an anagram indicator.
'trains'+'set'='trainsset'
'trainsset' is an anagram of 'RESISTANT'.
(Other definitions for resistant that I've seen before include "unlikely to be damaged?" , "not accommodating" , "Unyielding - proof" , "Not affected by something, a bug by an antibiotic say" , "Rebel" .)