Work with uranium, fast becoming rich (7)
I believe the answer is:
opulent
'rich' is the definition.
(similar in meaning)
'work with uranium fast' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'with' says to put letters next to each other.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'op'+'u'+'lent'='OPULENT'
'becoming' is the link.
(Other definitions for opulent that I've seen before include "Wealthy or luxuriant" , "Well-to-dp. affluent" , "Rich, luxuriant" , "Ritzy" , "Wealthy, got lots of money" .)