But on Tyneside it means a sandwich (5)
I believe the answer is:
butty
'a sandwich' is the definition.
(I've seen this before)
'but on tyneside it' is the wordplay.
'on' is a charade indicator (letters next to each other).
'tyneside' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'y' put after 't' is 'ty'.
'but'+'ty'='BUTTY'
'means' acts as a link.
Can you help me to learn more?
(Other definitions for butty that I've seen before include "something to eat" , "Sandwich; freight barge" , "Sandwich; towed barge" , "Partner" .)