Head of mule or donkey found in marsh (6)
I believe the answer is:
morass
'marsh' is the definition.
(I've seen this in another clue)
'head of mule or donkey' is the wordplay.
'head of' suggests taking the first letters.
'donkey' becomes 'ass' (ass is a term for a donkey).
The first letter of 'mule' is 'm'.
'm'+'or'+'ass'='MORASS'
'found in' acts as a link.
(Other definitions for morass that I've seen before include "Slough - marsh" , "Area of marshy or swampy ground" , "Tract of soft, wet ground" , "Muddled situation" , "Mire, quagmire" .)