Fen country wife has dental treatment? (7)
I believe the answer is:
wetland
'fen country' is the definition.
(fen is a type of wetland)
'wife has dental treatment?' is the wordplay.
'wife' becomes 'w'.
'has' says to put letters next to each other.
'treatment?' indicates anagramming the letters (I've seen 'treatment' mean this).
'dental' anagrammed gives 'etland'.
'w'+'etland'='WETLAND'
(Other definitions for wetland that I've seen before include "Marshy area" , "Boggy territory" .)