List of numbers shrinking quietly over time (6,5)
I believe the answer is:
yellow pages
'list of numbers' is the definition.
(book listing phone numbers)
'shrinking quietly over time' is the wordplay.
'shrinking' becomes 'yellow' (both can mean cowardly).
'quietly' becomes 'p' (p for piano and quiet).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'time' becomes 'ages' (ages can informally mean a long period).
'yellow'+'p'+'ages'='YELLOW PAGES'
(Other definitions for yellow pages that I've seen before include "Telephone reference book" , "looking for service here?" , "Reference book for business numbers" , "Directory" .)