They keep you feet dry (in wells, that is) (7)
I believe the answer is:
wellies
'they keep you feet dry' is the definition.
The definition suggests an adverb but the answer is not.
'in wells that is' is the wordplay.
'in' is an insertion indicator.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'wells' going around 'ie' is 'WELLIES'.
Can you help me to learn more?
(Other definitions for wellies that I've seen before include "provincial students in the main" , "Muddy footwear?" , "Rubber boots (colloq.)" .)