One left money for the sick (3)
I believe the answer is:
ill
'the sick' is the definition.
(I've seen this before)
'one left money' is the wordplay.
'one' becomes 'i' (Roman numeral).
'left' becomes 'L' (common abbreviation).
'money' becomes 'l' (L can mean 'pounds').
'i'+'l'+'l'='ILL'
'for' is the link.
(Other definitions for ill that I've seen before include "Evil or sick" , "Afflicted" , "'Never speak . . . of the dead'" , "Not well, sick" , "In bad health" .)