Perhaps use caution — departing unionist is stubborn (9)
I believe the answer is:
tenacious
'stubborn' is the definition.
(similar in meaning)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for tenacious that I've seen before include "Keeping a firm hold" , "refusing to give up" , "Holding fast" , "Dogged" , "Holding or sticking firmly" .)