Girl going by jet or prison van (5,5)
I believe the answer is:
black maria
'prison van' is the definition.
'black Maria' can be an answer for 'van' (black Maria is a kind of van). I'm not certain of the 'prison' bit.
'girl going by jet' is the wordplay.
'girl going' becomes 'maria' (I can't explain this - if you can you should give a lot more credence to this answer).
'by' is a charade indicator (letters next to each other).
'jet' becomes 'black' (similar in meaning).
'maria' after 'black' is 'BLACK MARIA'.
'or' acts as a link.
(Other definitions for black maria that I've seen before include "Nick vehicle" , "Police prisoners' transport" , "vehicle for criminals" , "Old police van for transporting prisoners" , "Van with criminals" .)