Stronger sack carrying money (roubles) (6)
I believe the answer is:
firmer
'stronger' is the definition.
('firm' can be a synonym of 'strong')
'sack carrying money roubles' is the wordplay.
'sack' becomes 'fire' (synonyms).
'carrying' means one lot of letters goes inside another.
'money' becomes 'm' (abbreviation in economics).
'roubles' becomes 'R'.
'fire' placed around 'm' is 'firme'.
'firme'+'r'='FIRMER'
(Other definitions for firmer that I've seen before include "Steadier" , "harder to uproot?" , "Stiffer" , "More resolute" , "not so easy to penetrate?" .)