Siren goes off on introduction of arm ball (9)
I believe the answer is:
inswinger
'ball' is the definition.
(I've seen this before)
'siren goes off on introduction of arm' is the wordplay.
'goes off' indicates anagramming the letters.
'on' is an insertion indicator (as in clothing 'on' a person).
'introduction of arm' becomes 'wing' (I can't justify this - if you can you should believe this answer much more).
'siren' with letters rearranged gives 'inser'.
'inser' placed around 'wing' is 'INSWINGER'.
Can you help me to learn more?
(Other definitions for inswinger that I've seen before include "Swerving ball" , "swerving delivery" , "ball from fast bowler" , "delivery curling through air" , "moving ball" .)