Fantasist rearmed going wild (7)
I believe the answer is:
dreamer
'fantasist' is the definition.
(I've seen this before)
'rearmed going wild' is the wordplay.
'going wild' indicates anagramming the letters.
'rearmed' with letters rearranged gives 'DREAMER'.
(Other definitions for dreamer that I've seen before include "Martin Luther King" , "One who tends to live in an unreal world" , "One fantasising" , "such an unpractical person!" , "Romantic" .)