a suit of clothes 做主语,谓语是单数
某些表示用具的复数名词glasses,clothes,trousers 做主语时,谓语用复数.
E.g.My trousers are being washed.Where are your glasses?
但是,如果前面有a / the pair of；a / the suit of等词语时,则谓语要用单数.
E.g.That pair of glasses was lying on the desk.---A suit of clothes was lying on the table.