Crack around top of head produces pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'crack around top of head' is the wordplay.
'crack' becomes 'ace' (I've seen this before).
'around' is an insertion indicator.
'top of' suggests taking the first letters.
The initial letter of 'head' is 'h'.
'ace' enclosing 'h' is 'ACHE'.
'produces' acts as a link.
(Other definitions for ache that I've seen before include "Suffer dull pain" , "Hurt; yearn" , "Persistent dull pain" , "Pine" , "A dull lingering pain" .)