Silver following doctor's trail (4)
I believe the answer is:
drag
'trail' is the definition.
(I've seen this before)
'silver following doctor's' is the wordplay.
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'following' means one lot of letters go next to another.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'ag' after 'dr' is 'DRAG'.
(Other definitions for drag that I've seen before include "Pull along roughly" , "Short smoke" , "Yank" , "Women's clothes worn by a man - pull" , "Haul - queen's garb?" .)