Parking inside not very well but fast (6)
I believe the answer is:
speedy
'fast' is the definition.
('speedy' can be similar in meaning to 'fast')
'parking inside not very well' is the wordplay.
'parking' becomes 'p'.
'inside' indicates putting letters inside.
'not very well' becomes 'seedy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'p' put inside 'seedy' is 'SPEEDY'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for speedy that I've seen before include "Expeditious" , "Swift" , "Fast-moving" , "'Fast, without delay (6)'" , "in a hurry" .)