Sir Thomas's quarry bagged by our opponents, sadly (5,3,4)
I believe the answer is:
mores the pity
'sir' is the definition.
Both the answer and definition are singular nouns.
Maybe they are linked in a way I don't understand?
'thomas's quarry bagged by our opponents sadly' is the wordplay.
I cannot really see how this works, but
'quarry' could be 'pit' ('pit' can be a synonym of 'quarry') and 'pit' is found within the answer.
'opponents' could be 'es' (I've seen this in other clues) and 'es' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for mores the pity that I've seen is " And that's regrettable".)