Fiat car has a little difficulty in reverse (4)
I believe the answer is:
undo
'reverse' is the definition.
(undoing is a kind of reversing)
'fiat car has a little difficulty' is the wordplay.
'fiat car' becomes 'uno' (I can't explain this - if you can you should believe this answer much more).
'has' indicates putting letters inside.
'a little' says to take the initial letters.
The first letter of 'difficulty' is 'd'.
'uno' enclosing 'd' is 'UNDO'.
'in' is the link.
Can you help me to learn more?
(Other definitions for undo that I've seen before include "Subvert" , "Release fastening" , "Cause the ruin of, or cancel" , "Open button or ruin" , "Loosen; annul" .)