Delay follows parking jam (8)
I believe the answer is:
preserve
'jam' is the definition.
(jam is a type of preserve)
'delay follows parking' is the wordplay.
I cannot quite see how this works, but
'parking' could be 'p' (abbreviation on road signs) and 'p' is present in the answer.
The remaining letters 'reserve' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for preserve that I've seen before include "Prevent from deteriorating" , "Maintain; keep safe" , "kipper?" , "Jam or marmalade" , "Maintain in unaltered condition" .)