Doctor Silver is a bore (4)
I believe the answer is:
drag
'a bore' is the definition.
(thesaurus)
'doctor silver' is the wordplay.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'dr'+'ag'='DRAG'
'is' is the link.
(Other definitions for drag that I've seen before include "Haul - a bore!" , "Draw slowly or heavily" , "Trail; bore" , "Pull along the ground - transvestite'-'s get-up" , "Transvestite's garb" .)