The bar is back: have a great time (5)
I believe the answer is:
revel
'have a great time' is the definition.
(to revel is to party or celebrate)
'the bar is back' is the wordplay.
'the bar' becomes 'lever' (lever is a kind of bar).
'is back' shows that the letters should be reversed in order.
'lever' reversed gives 'REVEL'.
(Other definitions for revel that I've seen before include "Celebrate with a lever, strangely" , "Take great pleasure" , "Be festive, enjoy" , "Take great delight (in)" , "Rejoice" .)