They keep your feet dry, that is, in watery places (7)
I believe the answer is:
wellies
'watery places' is the definition.
The definition suggests a plural noun which matches the answer.
'they keep your feet dry that is' is the wordplay.
I cannot really understand how this works, but
'that is' could be 'ie' (short for Latin 'id est', 'that is') and 'ie' is found in the answer.
The remaining letters 'wells' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for wellies that I've seen before include "Muddy footwear?" , "Rubber boots (colloq.)" , "provincial students in the main" .)