Wild flare-up (5)
I believe the answer is:
feral
'wild' is the definition.
(similar in meaning)
'flare-up' is the wordplay.
'up' is an anagram indicator.
'flare' with letters rearranged gives 'FERAL'.
(Other definitions for feral that I've seen before include "Fierce, having reverted to wild state" , "Escaped-into-wild (animal)" , "Wild or brutal" , "Flare up, being fierce" , "Wild, untamed" .)