Disgusting diet ending in February (5)
I believe the answer is:
slimy
'disgusting' is the definition.
(thesaurus)
'diet ending in february' is the wordplay.
'diet' becomes 'slim' (both can mean to attempt to lose weight).
'ending in' indicates one should take the final letters.
The final letter of 'february' is 'y'.
'slim'+'y'='SLIMY'
(Other definitions for slimy that I've seen before include "toadying" , "Moist and slippery" , "Repulsively obsequious" , "Covered with slippery stuff, say mud" , "Glutinous" .)