Flare up from a muscle rupture (5)
I believe the answer is:
erupt
'flare up' is the definition.
('erupt' can be a synonym of 'flare up')
'from a muscle rupture' is the wordplay.
'from' says the answer is hidden in the clue.
'ERUPT' is hidden in the letters of 'a muscle rupture'.
(Other definitions for erupt that I've seen before include "Break through (as a volcano)" , "Emit lava" , "Explode" , "Break out like Etna" , "Spurt lava" .)