Fast getting tied in a knot! (4)
I believe the answer is:
diet
'fast' is the definition.
(dieting is a kind of fasting)
'tied in a knot' is the wordplay.
'in a knot' indicates an anagram.
'tied' anagrammed gives 'DIET'.
'getting' acts as a link.
(Other definitions for diet that I've seen before include "Attempt to shift a few pounds" , "provisions" , "Worms once a source of this" , "Prescribed selection of foods" , "Food and drink regularly consumed" .)