Shrink fleece when daughter takes son's place (7)
I believe the answer is:
dwindle
'shrink' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for dwindle that I've seen before include "Grow less - waste away" , "Decline" , "Peter out" , "(Flow) decrease" , "Become smaller or fewer" .)