Quick means of communication vital to the diver
I believe the answer is:
airline
'quick' is the definition.
Both the answer and definition are adjectives.
Perhaps there's an association between them I don't understand?
'means of communication vital to the diver' is the wordplay.
I cannot really see how this works, but
'means' could be 'air' (I have seen 'Means of travelling' mean 'air ' so perhaps 'means' could also mean 'air ') and 'air' is present in the answer.
'communication' could be 'line' (line is a kind of communication) and 'line' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for airline that I've seen before include "Aircraft operating company" , "Virgin, possibly" , "Qantas, for example" , "Company operating aeroplanes" , "It serves both divers and travellers" .)