Festivities in bar turn extremely rowdy (7)
I believe the answer is:
revelry
'festivities' is the definition.
(I've seen this before)
'bar turn extremely rowdy' is the wordplay.
'bar' becomes 'lever' (lever is a kind of bar).
'turn' shows that the letters should be reversed in order.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'rowdy' with its middle removed is 'ry'.
'lever' reversed gives 'revel'.
'revel'+'ry'='REVELRY'
'in' is the link.
(Other definitions for revelry that I've seen before include "Lively celebrations" , "Feasting, noisy festivities" , "Noisy celebration" , "Jolly parties" , "Riotous feasting" .)