Woman crossing ends of fields much like the Fens (6)
I believe the answer is:
marshy
'like the fens' is the definition.
'woman crossing ends of fields much' is the wordplay.
'woman' becomes 'mary' (**).
'crossing' is an insertion indicator.
'ends of' suggests the final letters (I've seen 'end' mean this).
The last letters of 'fields much' are 'sh'.
'mary' enclosing 'sh' is 'MARSHY'.
(Other definitions for marshy that I've seen before include "Long narrow mark" , "Like swamps" , "Boggy; waterlogged" , "treacherous underfoot" , "Like fenlands" .)