Old cow marshal observed at foot of tree (8)
I believe the answer is:
alderney
'old cow marshal' is the definition.
The definition suggests a singular noun which matches the answer.
'observed at foot of tree' is the wordplay.
'observed' becomes 'ney' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'at foot of' means one lot of letters go next to another (in a down clue, some letters go below others).
'tree' becomes 'alder' (alder is a kind of tree).
'ney' put after 'alder' is 'ALDERNEY'.
Can you help me to learn more?
(Other definitions for alderney that I've seen before include "Third largest of the Channel Islands" , "A Channel Island" , "Channel isle" , "Cattle" , "Cow, perhaps" .)