Doctor finding headless fish in swamp (6)
I believe the answer is:
drench
'swamp' is the definition.
(synonyms)
'doctor finding headless fish' is the wordplay.
'doctor finding' becomes 'dr' (I am not sure about this - if you are sure you should believe this answer much more).
'headless' means to remove the first letter.
'fish' becomes 'tench' (tench is a kind of fish).
'tench' with its initial letter removed is 'ench'.
'dr'+'ench'='DRENCH'
'in' is the link.
(Other definitions for drench that I've seen before include "Soak, souse" , "draught?" , "Soak or saturate" , "Swamp" , "Wet thoroughly, soak" .)