Greed — A weakness that engulfed a king (7)
I believe the answer is:
avarice
'greed' is the definition.
(avarice is a kind of greed)
'a weakness that engulfed a king' is the wordplay.
'weakness' becomes 'vice' (a moral weakness).
'that engulfed' indicates putting letters inside.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'a'+'r'='ar'
'vice' going around 'ar' is 'varice'.
'a'+'varice'='AVARICE'
(Other definitions for avarice that I've seen before include "Insatiable desire for wealth" , "Excessive greed for wealth" , "Wanting to be rich" , "Greed - caviare (anag)" , "Parsimony" .)