Brown monkeys climb all around one (5)
I believe the answer is:
sepia
'brown' is the definition.
(I know this)
'monkeys climb all around one' is the wordplay.
'monkeys' becomes 'apes' (I've seen this in other clues).
'climb' shows that the letters should be reversed in order (I've seen 'climbing' mean this (in a down clue, letters climb up)).
'all around' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'apes' backwards is 'sepa'.
'sepa' enclosing 'i' is 'SEPIA'.
(Other definitions for sepia that I've seen before include "source of ink" , "Old drawing" , "Former photographic pigment" , "brownish colour" , "Brown colour of old photo" .)