In the middle of day, dressed in silver (5)
I believe the answer is:
among
'in the middle of' is the definition.
(I've seen this in another clue)
'day dressed in silver' is the wordplay.
'day' becomes 'mon' (short for Monday).
'dressed in' is an insertion indicator (inserted letters are dressed in others).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'mon' placed into 'ag' is 'AMONG'.
(Other definitions for among that I've seen before include "Go man! (anag.)" , "in a group" , "Included with - inside" , "Amid" , "when in company" .)