Miss reverse gear, and fail to finish (4)
I believe the answer is:
girl
'miss' is the definition.
('girl' can be a synonym of 'miss')
'reverse gear and fail to finish' is the wordplay.
'reverse' says the letters should be written in reverse.
'gear' becomes 'rig' (rig is a kind of gear).
'and' means one lot of letters go next to another.
'to finish' says to take the final letters.
The last letter of 'fail' is 'l'.
'rig' backwards is 'gir'.
'gir'+'l'='GIRL'
(Other definitions for girl that I've seen before include "Maid" , "Young woman" , "One of 8 [LASSES]" , "Maidservant" , "Young lady" .)