Ruminant, note, observed in climbing valley (5)
I believe the answer is:
eland
'ruminant' is the definition.
(eland is a kind of ruminant)
'note observed in climbing valley' is the wordplay.
'note' becomes 'n' (abbreviation).
'observed in' is an insertion indicator.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'valley' becomes 'dale' (dale is a kind of valley).
'dale' written backwards gives 'elad'.
'n' placed into 'elad' is 'ELAND'.
(Other definitions for eland that I've seen before include "Browser" , "Large spiral-horned African antelope" , "wild creature" , "Laden (anag.); antelope" , "Ruminant" .)