Old soak needs small operation (3)
I believe the answer is:
sop
'old soak' is the definition.
(both can mean a drunk)
'small operation' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'operation' becomes 'op'.
's'+'op'='SOP'
'needs' is the link.
(Other definitions for sop that I've seen before include "Appeasing gift" , "Appeasing bribe" , "Something of no great value given to appease" , "Small concession" , "Concession given to mollify or placate" .)