Sickly after vacation, blames bad diet (8)
I believe the answer is:
assembly
'diet' is the definition.
(thesaurus)
'sickly after vacation blames bad' is the wordplay.
'after vacation' says to hollow out the word (remove centre letters) (the inside letters must vacate).
'bad' indicates anagramming the letters.
'sickly' with its centre removed is 'sy'.
'sy'+'blames'='syblames'
'syblames' anagrammed gives 'ASSEMBLY'.
(Other definitions for assembly that I've seen before include "This line is a sequence of machines and workers" , "A gathering for formal meeting" , "court" , "start of day at school" , "Bless May (anag.)" .)