Five Italians, not half lively (5)
I believe the answer is:
vital
'lively' is the definition.
('vital' can be similar in meaning to 'lively')
'five italians not half' is the wordplay.
'five' becomes 'v' (roman numeral).
'not half' means to take half the letters.
'italians' cut in half is 'ital'.
'v'+'ital'='VITAL'
(Other definitions for vital that I've seen before include "Crucially important, essential" , "Essential to life, indispensable" , "Characteristic of life" , "Most important, like certain body organs" , "Essential; lively" .)