How a pin can hurt! (4)
I believe the answer is:
pain
'hurt' is the definition.
(I know that pain is a kind of hurt)
'how a pin' is the wordplay.
'how' is an anagram indicator (I've seen this in other clues).
'a'+'pin'='apin'
'apin' anagrammed gives 'PAIN'.
'can' is the link.
(Other definitions for pain that I've seen before include "Agony" , "Tiresome figure" , "French bread?" , "Suffering, ache" , "Sensation of acute discomfort" .)