Correctly positioned on motorway when lorry at the front pulled out — ridiculous! (5)
I believe the answer is:
inane
'ridiculous' is the definition.
(both can mean silly)
'correctly positioned on motorway when lorry at the front pulled out' is the wordplay.
I cannot quite see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
'at' could be 'in' and 'in' is present in the answer.
The remaining letters 'an' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for inane that I've seen before include "Lacking sense, silly" , "Stupid and silly" , "Empty" , "Thick" , "Vacuous, silly" .)