Once a mighty worker, full of life (9)
I believe the answer is:
exuberant
'full of life' is the definition.
(exuberant means full of energy or life)
'once a mighty worker' is the wordplay.
'once' becomes 'ex'.
'a mighty' becomes 'uber' (uber can mean great or extreme).
'worker' becomes 'ant' (worker is a type of ant).
'ex'+'uber'+'ant'='EXUBERANT'
(Other definitions for exuberant that I've seen before include "Enthusiastic" , "Abounding in vitality" , "full of glee" , "Joyous" , "Full of energy, high spirits" .)