One who pinches a young child (6)
I believe the answer is:
nipper
'young child' is the definition.
(I've seen this before)
'one who pinches a' is the wordplay.
'one who pinches' becomes 'nip' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'nip'+'per'='NIPPER'
Can you help me to learn more?
(Other definitions for nipper that I've seen before include "Young person (with claws?)" , "One pinches" , "Claw (large) - child (small)" , "Child (informal); one biting" , "one given to pinching things" .)