Swallows fast in undulating hills (5)
I believe the answer is:
This might be a double definition.
'swallows' is the first definition.
(I've seen this in another clue)
'hills' is the second definition.
(I've seen this before)
'fast in undulating' acts as a link.
I am not very happy about this link. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for downs that I've seen before include "Drinks in one" , "Knocks over; drinks" , "Gently rolling hills" , "Southern uplands" , "North or South, two features of chalk upland in England" .)