Very happy once with taxi worker (9)
I believe the answer is:
exuberant
'very happy' is the definition.
(exuberant means very happy or excited)
'once with taxi worker' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'with' is a charade indicator (letters next to each other).
'taxi' becomes 'uber'.
'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" , "Joyous" , "Lively, cheerful" , "Elated" .)