Charlie and Jack in driver's compartment (3)
I believe the answer is:
cab
'driver's compartment' is the definition.
(I've seen this in another clue)
'charlie and jack' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet: alpha, bravo, charlie etc.).
'and' means one lot of letters go next to another.
'jack' becomes 'ab' (both can mean a sailor).
'c'+'ab'='CAB'
'in' acts as a link.
(Other definitions for cab that I've seen before include "Transport" , "'The authority collects money made from crime, initially (3)'" , "Hackney" , "Driving compartment" , "A taxi in New York" .)