Like certain teenage turtles (6)
I believe the answer is:
mutant
'like certain teenage turtles' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mutant that I've seen before include "Living thing that has undergone genetic change" , "Genetically changed form" , "badly bred" , "Genetically-modified" , "Organism that has diverged from type" .)