Assassin's not at all sweet unless on vacation (6)
I believe the answer is:
brutus
'assassin's' is the definition.
'not at all sweet unless on vacation' is the wordplay.
'not at all sweet' becomes 'brut' (I've seen this before).
'on vacation' says to hollow out the word (remove centre letters) (after the inside letters have vacated the word).
'unless' with its middle taken out is 'us'.
'brut'+'us'='BRUTUS'
(Other definitions for brutus that I've seen before include "Murderer of Caesar" , "Conspirator's" , "An honourable man, said Antony" , "Conspirator in the murder of Julius Caesar" , "statesman who committed suicide in Macedonia" .)