Talk a lot in good Anglo-Saxon! (3)
I believe the answer is:
gas
'talk a lot' is the definition.
(gas can mean to talk at length)
'good anglo-saxon' is the wordplay.
'good' becomes 'g' (abbreviation).
'anglo-saxon' becomes 'AS' (abbreviation for Anglo-Saxon).
'g'+'as'='GAS'
'in' acts as a link.
(Other definitions for gas that I've seen before include "eg CO2" , "Nitrogen maybe" , "Eg, nitrogen" , "What's needed to drive" , "idle chit-chat" .)