Quick way to get slim (4)
I believe the answer is:
fast
'quick' is the definition.
(I know that quick can be written as fast)
'way to get slim' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'to get' is a charade indicator (letters next to each other).
'slim' becomes 'fa' (I can't explain this - if you can you should give a lot more credence to this answer).
'st' put after 'fa' is 'FAST'.
Can you help me to learn more?
(Other definitions for fast that I've seen before include "Can't move" , "debauched" , "Secure; speedy" , "Period without food" , "requirement before surgery" .)