Clue: Tea before 5 on vacation, very definitely not 4 (6)
Answer: CHAVVY
Explanation: Definition: definitely not 4
at least in the online version, the clue has 'defnitely' instead of 'def**i**nitely' 
definition: not the POSHEST (4dn). My Chambers has CHAV as "Someone who appears to have access to money but not necessarily to taste" – in my experience it is used to describe a certain type/caricature of young working-class people
 
CHA="tea" + V (5, in Roman numerals) + V-er-Y 'vacated' of its inner letters