Navigational aid for a lady in a posh car (5)
I believe the answer is:
radar
'navigational aid' is the definition.
(I've seen this before)
'lady in a posh car' is the wordplay.
'lady' becomes 'ada' ().
'in' means one lot of letters goes inside another.
'a posh car' becomes 'rr' (short for Rolls Royce).
'ada' going inside 'rr' is 'RADAR'.
'for a' acts as a link.
(Other definitions for radar that I've seen before include "Means of detecting and locating distant objects" , "Range-finding apparatus" , "Navigation aid" , "Method of detection" , "M.A.S.H. character; electronic range detector" .)