Very productive expert has most of career in charge (8)
I believe the answer is:
prolific
'very productive' is the definition.
(I've seen this in another clue)
'expert has most of career in charge' is the wordplay.
'expert' becomes 'pro' (short for professional).
'has' is a charade indicator (letters next to each other).
'most of' means to remove the last letter.
'career' becomes 'life' (career can mean someone's life or past).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'life' with its last letter taken away is 'lif'.
'pro'+'lif'+'ic'='PROLIFIC'
(Other definitions for prolific that I've seen before include "Abundantly productive" , "Plentiful" , "Abounding" , "Rich" , "Intellectually productive" .)