Criminal's offspring is going into painting (8)
I believe the answer is:
arsonist
'criminal's' is the definition.
(arsonist is a kind of criminal)
'offspring is going into painting' is the wordplay.
'offspring' becomes 'son' (son is a kind of offspring**).
'is going' becomes 'is' (I've seen this in other clues).
'into' means one lot of letters goes inside another.
'painting' becomes 'art' (I've seen this before).
'son'+'is'='sonis'
'sonis' put inside 'art' is 'ARSONIST'.
(Other definitions for arsonist that I've seen before include "So strain (anag.)" , "A major criminal" , "One firing property" , "Guy seeking to raise temperature" , "Like blazes he will" .)