One single Spanish vehicle, moving back, is lost (2,3)
I believe the answer is:
at sea
'is lost' is the definition.
(both can mean confused or in trouble)
'one single spanish vehicle moving back' is the wordplay.
I cannot quite see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
'single' could be 'a' and 'a' is found within the answer.
The remaining letters 'tse' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for at sea that I've seen before include "On a ship (out of one's depth?)" , "on a boat?" , "Confused, uncertain" , "Bewildered on the ocean?" , "Totally disorganised" .)