Like the Fens, ultimately rather quiet in May (6)
I believe the answer is:
marshy
'like the fens' is the definition.
(the Fens is a marshy region)
'ultimately rather quiet in may' is the wordplay.
'ultimately' says to take the final letters.
'quiet' becomes 'sh'.
'in' indicates putting letters inside.
The last letter of 'rather' is 'r'.
'r'+'sh'='rsh'
'rsh' going into 'may' is 'MARSHY'.
(Other definitions for marshy that I've seen before include "Soggy" , "Damp and low-lying" , "Boggy, swampy" , "Wet and low-lying" , "(Of land) waterlogged" .)