It's painful when penny's pinched from pocket! (4)
I believe the answer is:
ouch
'it's painful' is the definition.
(expression of pain)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ouch that I've seen before include "Expression of pain" , "Exclamation of sudden pain" , "Shout of pain" , "Cry of pain" , "Pained expression" .)