Men backed replacing leader of Sir David's old constituencies (6,8)
I believe the answer is:
rotten boroughs
'constituencies' is the definition.
Although both the answer and definition are plural nouns, I don't see how they can define each other.
'men backed replacing leader of sir david's old' is the wordplay.
I cannot quite see how this works, but
'men' could be 'or' (military abbreviation for Other Ranks) and 'or' is located in the answer.
'leader' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
'of' could be 'o' and 'o' is found within the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?