Who will back us for £50 against that terrible Russian? Not Gilbert! (8)
I believe the answer is:
sullivan
'back us for ps50 against that terrible russian? not gilbert' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe there's an association between them I don't understand?
'who will' is the wordplay.
I cannot quite see how this works, but
'will' could be 'll' (contracted form, as in 'he'll' - 'he will') and 'll' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sullivan that I've seen before include "Mikado composer" , "Savoy Opera composer" , "Sir 4 ---, composer" , "The S in G&S" , "Composer of music for Ruddigore, d. 1900" .)