Radical baseball pitcher (6)
I believe the answer is:
leftie
'radical' is the definition.
(a left-wing person)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for leftie that I've seen before include "supporter of "gauche"?" , "A radical (colloq.)" , "socialist" , "Person with radical tendencies (informal)" , "Red" .)