Gateshead girl became a tumbler (5)
I believe the answer is:
glass
'a tumbler' is the definition.
(I've seen this before)
'gateshead girl' is the wordplay.
'gateshead' becomes 'g' (head letter of 'gates').
'girl' becomes 'lass' (lass is a kind of girl**).
'g'+'lass'='GLASS'
'became' is the link.
(Other definitions for glass that I've seen before include "Drinking vessel" , "Greenhouse material" , "Philip --, US composer" , "Vitreous material" , "such as one might hold at party" .)