Ready to drive off, wearing harness (2,4)
'ready to drive off' is the definition.
(I've seen this before)
'wearing harness' is the wordplay.
'wearing' becomes 'in' (wear in (and 'in' is fashionable)).
'harness' becomes 'gear' (a harness is the gear of a working animal).
(Other definitions for in gear that I've seen before include "Performing effectively", "Out of neutral?", "Ready to move (of car)", "ready to go?", "Working effectively".)