Reddish-brown primates climbing around one (5)
I believe the answer is:
sepia
'reddish-brown' is the definition.
(I've seen this before)
'primates climbing around one' is the wordplay.
'primates' becomes 'apes' (ape is a kind of primate).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'around' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'apes' written backwards gives 'sepa'.
'sepa' going around 'i' is 'SEPIA'.
(Other definitions for sepia that I've seen before include "Dark reddish-brown pigment" , "Brownish tone associated with old photographs" , "Colourant" , "Denizen of deep" , "Brown with a red tinge" .)