Talk on 50A light (3,4)
I believe the answer is:
gas lamp
'light' is the definition.
(gas lamp is a kind of light)
'talk on 50a' is the wordplay.
'talk' becomes 'gas' (gas can mean to chat).
'on' means one lot of letters go next to another.
'50a' becomes 'lamp' (I am not sure about this - if you are sure you should believe this answer much more).
'gas'+'lamp'='GAS-LAMP'
(Other definitions for gas lamp that I've seen before include "may be illuminating" , "Old illuminator" , "Sort of light" , "Bright object" , "lighter" .)