Get better following diet strictly (9)
I believe the answer is:
literally
'strictly' is the definition.
(thesaurus)
'get better following diet' is the wordplay.
'get better' becomes 'rally' (I've seen this before).
'following' says to put letters next to each other.
'diet' becomes 'lite' ('lite' can be a synonym of 'diet').
'rally' put after 'lite' is 'LITERALLY'.
(Other definitions for literally that I've seen before include "Without exaggeration" , "Exactly as stated" , "According to the letter, exactly" , "precisely as written" , "According to the precise meaning of words" .)