Exclamation of surprise about old language in Northern river? (6)
I believe the answer is:
mersey
'northern river?' is the definition.
(river in the North West of England)
'exclamation of surprise about old language' is the wordplay.
'exclamation of surprise' becomes 'my'.
'about' indicates putting letters inside.
'old language' becomes 'erse' (old-fashioned term for Gaelic).
'my' going around 'erse' is 'MERSEY'.
'in' is the link.
(Other definitions for mersey that I've seen before include "River with ferry and tunnel" , "User met Lenny (anag) - going between Liverpool and Birkenhead" , "River linking Greater Manchester and Liverpool" , "Scouse river" , "River rising in Derbyshire and flowing into the Irish Sea" .)