Trains so curiously to be a criminal (8)
I believe the answer is:
arsonist
'a criminal' is the definition.
(I know that arsonist is a type of criminal)
'trains so curiously' is the wordplay.
'curiously' indicates anagramming the letters.
'trains'+'so'='trainsso'
'trainsso' is an anagram of 'ARSONIST'.
'to be' is the link.
(Other definitions for arsonist that I've seen before include "Fire raiser" , "Criminal fire-raiser" , "A major criminal" , "Incendiary, firebug" , "Guy seeking to raise temperature" .)