One who pinches a child (6)
I believe the answer is:
nipper
'child' is the definition.
(I know that child can be written as nipper)
'one who pinches a' is the wordplay.
'one who pinches' becomes 'nip' (I can't explain this - if you can 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 "Kid - claw" , "Little kid" , "Infant (colloq.)" , "Crab - young shaver?" , "Child - ankle-biter?" .)