Use of a fire engine (9)
I believe the answer is:
appliance
I believe this is a double definition.
'use' is the first definition.
(thesaurus)
'fire engine' is the second definition.
(I've seen this before)
'of a' is the link.
(Other definitions for appliance that I've seen before include "Instrument for a particular purpose" , "Piece of kit" , "Tool or device for a specific task" , "Domestic machine" , "Device performing specific task" .)