Slim until you fade away at the treatment centre (4)
I believe the answer is:
diet
'slim' is the definition.
(thesaurus)
'fade away at the treatment centre' is the wordplay.
'fade away' becomes 'die' (I've seen this in other clues).
'at' means one lot of letters go next to another.
'centre' indicates the central letters.
The middle letter of 'treatment' is 't'.
'die'+'t'='DIET'
'until you' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for diet that I've seen before include "Weight-loss plan" , "Restricted intake" , "Special food regime" , "Restricted cheer" , "Worms had one" .)