What does the cryptic crossword clue "Whistler's Mother above shelf when last viewed (3)" mean?

The definition is "Whistler". The wordplay uses acrostic. last letters of "Mother above shelf" = REF; definition: "Whistler's" The answer is REF.

Whistler's Mother above shelf when last viewed (3)

Times Cryptic #28769 | 25 Down | 2023-11-24 flower