Work by Updike initially fast, becoming rich (7)
I believe the answer is:
opulent
'rich' is the definition.
('opulent' can be similar in meaning to 'rich')
'work by updike initially fast' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'by' says to put letters next to each other.
'initially' says to take the initial letters.
'fast' becomes 'lent' (I've seen this before).
The initial letter of 'updike' is 'u'.
'op'+'u'+'lent'='OPULENT'
'becoming' is the link.
(Other definitions for opulent that I've seen before include "No let-up (anag)" , "Ostentatiously costly and luxurious" , "Rolling" , "'Deluxe, luxurious (7)'" , "Wealthy or luxuriant" .)