Stay in vehicle number eleven (4)
I believe the answer is:
busk
'stay' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's an association between them I don't understand?
'vehicle number eleven' is the wordplay.
'vehicle' becomes 'bus' (bus is a kind of vehicle).
'number eleven' becomes 'k' (eleventh letter of the alphabet).
'bus'+'k'='BUSK'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for busk that I've seen before include "provide entertainment for money" , "Play for donations" , "Prepare" , "Improvise; perform for money in the street" , "Play street music" .)