He'll wait until the trip is over (7)
I believe the answer is:
steward
'he'll wait until' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'the trip is over' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'trip' becomes 'seward' (I can't justify this - if you can you should give a lot more credence to this answer).
'is over' is an insertion indicator.
't' inserted into 'seward' is 'STEWARD'.
Can you help me to learn more?
(Other definitions for steward that I've seen before include "Caretaker" , "Aircraft attendant" , "Horse-racing official" , "Ship's waiter" , "Official at event" .)