Satisfied, endlessly wealthy in such a system (6)
I believe the answer is:
metric
'such a system' is the definition.
(I have seen 'System of measurement' mean 'metric' so perhaps 'system' could also mean 'metric')
'satisfied endlessly wealthy' is the wordplay.
'satisfied' becomes 'met' (as in 'met the conditions of his contract').
'endlessly' means to remove the last letter.
'wealthy' becomes 'rich' (similar in meaning).
'rich' with its last letter taken off is 'ric'.
'met'+'ric'='METRIC'
'in' acts as a link.
(Other definitions for metric that I've seen before include "Like the SI system" , "System using litre and gram as units" , "Quantitative" , "Successor to imperial (system)" , "Like decimal measuring system" .)