Money can corrupt, ultimately (4)
I believe the answer is:
loot
'money' is the definition.
(I've seen this before)
'can corrupt ultimately' is the wordplay.
'can' becomes 'loo' (loo is a kind of can**).
'ultimately' suggests the final letters.
The last letter of 'corrupt' is 't'.
'loo'+'t'='LOOT'
(Other definitions for loot that I've seen before include "Haul" , "Plunder and ransack" , "Steal property (in war)" , "Stolen goods" , "Steal (goods); money" .)