The most common and most likely answer to this crossword clue is the 4 letter word CARR
C
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More clues leading to the result CARR
Buddy explains!

The straight part
We know that "fen woodland" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fen woodland = carr.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CARR mean?
CARR noun- A bog or marsh; marshy ground, swampland.
- A marsh or fen on which low trees or bushes grow; a marshy woodland.
- A wheeled vehicle that moves independently, with at least three wheels, powered mechanically, steered by a driver and mostly for personal transportation.
- A wheeled vehicle, drawn by a horse or other animal; a chariot.
- An unpowered unit in a railroad train.
- An individual vehicle, powered or unpowered, in a multiple unit.
- A passenger-carrying unit in a subway or elevated train, whether powered or not.
- A rough unit of quantity approximating the amount which would fill a railroad car.
- The moving, load-carrying component of an elevator or other cable-drawn transport mechanism.
- The passenger-carrying portion of certain amusement park rides, such as Ferris wheels.
- The part of an airship, such as a balloon or dirigible, which houses the passengers and control apparatus.
- A sliding fitting that runs along a track.
- The aggregate of desirable characteristics of a car.
- A floating perforated box for living fish.