Lively function packed – one left out (8)
I believe the answer is:
eventful
'lively' is the definition.
('eventful' can be similar in meaning to 'lively')
'function packed one left out' is the wordplay.
'function' becomes 'event' (I've seen this before**).
'packed' becomes 'full' ('full' can be a synonym of 'packed').
'one left' becomes 'l' (common abbreviation).
'out' is a deletion indicator.
'full' with 'l' taken away is 'ful'.
'event'+'ful'='EVENTFUL'
(Other definitions for eventful that I've seen before include "Consequential" , "Full of incident, not dull" , "Memorable; momentous" , "Notable, full of incidents" , "Busy, exciting (period)" .)