Source of sharp pain? Repeats “Ooh!” when stricken (10)
I believe the answer is:
peashooter
'source of sharp pain?' is the definition.
(a peashooter is used to fire dried peas at someone)
'repeats ooh when stricken' is the wordplay.
'when stricken' indicates an anagram (letters are struck or hit).
'repeats'+'ooh'='repeatsooh'
'repeatsooh' with letters rearranged gives 'PEASHOOTER'.
(Other definitions for peashooter that I've seen before include "Primitive weapon" , "Toy weapon" , "Child's "gun"" , "child's toy" .)