Old silver starts to exhibit dullness (4)
I believe the answer is:
aged
'old' is the definition.
('aged' can be similar in meaning to 'old')
'silver starts to exhibit dullness' is the wordplay.
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'starts to' indicates taking the first letters.
The initial letters of 'exhibit dullness' is 'ed'.
'ag'+'ed'='AGED'
(Other definitions for aged that I've seen before include "Elderly" , "Got older" , "way over the hill" , "Senior" , "Geriatric" .)