A singular game for the relatively rotund (7)
I believe the answer is:
rounder
'a singular game for the relatively rotund' is the definition.
I can't judge whether this definition defines the answer.
'game for the relatively rotund' is the wordplay.
'game' becomes 'round' (round is a kind of game).
'for' says to put letters next to each other.
'the relatively rotund' becomes 'er' (I can't justify this - if you can you should give a lot more credence to this answer).
'round'+'er'='ROUNDER'
Can you help me to learn more?
(Other definitions for rounder that I've seen before include "More circular" , "Score at baseball-type game" , "complete circuit in a game" , "Or under (anag.)" , "More rotund" .)