Officer in boggy land by a lake (7)
I believe the answer is:
marshal
'officer' is the definition.
(I've seen this before)
'boggy land by a lake' is the wordplay.
'boggy land' becomes 'marsh'.
'by' means one lot of letters go next to another.
'lake' becomes 'l'.
'marsh'+'a'+'l'='MARSHAL'
'in' acts as a link.
(Other definitions for marshal that I've seen before include "Arrange in order" , "US law officer" , "US federal officer" , "High officer" , "guide" .)