They open tins or jars (6)
I believe the answer is:
intros
'they open' is the definition.
(intros begin or open pieces of music)
'tins or jars' is the wordplay.
'jars' is an anagram indicator (the letters must be shocked or jarred).
'tins'+'or'='tinsor'
'tinsor' with letters rearranged gives 'INTROS'.
(Other definitions for intros that I've seen before include "opening gestures" , "Opening passages" , "(Musical) lead-ins" .)