A vertical take off - honest! (8,2)
I believe the answer is:
straight up
'a vertical' is the definition.
(I've seen this before)
'off honest' is the wordplay.
I cannot quite understand how this works, but
'honest' could be 'straight' (similar in meaning) and 'straight' is found within the answer.
The remaining letters 'up' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'take' acts as a link.
Can you help me to learn more?
(Other definitions for straight up that I've seen before include "taking shortest route?" , "vertical" , "Exactly how one reaches summit" , "Getting to the top immediately" , "It's the quickest way to get high" .)