Let vicar out when upright (8)
I believe the answer is:
vertical
'upright' is the definition.
(I know that upright can be written as vertical)
'let vicar out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'let'+'vicar'='letvicar'
'letvicar' with letters rearranged gives 'VERTICAL'.
'when' is the link.
(Other definitions for vertical that I've seen before include "At right angles to the base line" , "At right angles to the horizontal" , "as is column" , "Ninety degrees to horizontal" , "Straight up" .)