Marsupial so quiet climbing on the whole? (6)
I believe the answer is:
possum
'marsupial' is the definition.
(possum is a kind of marsupial)
'so quiet climbing on the whole?' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'on' is a charade indicator (letters next to each other).
'the whole?' becomes 'sum' (sum is a kind of whole).
'so'+'p'='sop'
'sop' written backwards gives 'pos'.
'pos'+'sum'='POSSUM'
(Other definitions for possum that I've seen before include "Dame Edna fan?" , "Tree-dwelling Australasian marsupial" , "Marsupial - play it to feign sleep" , "Play -- (pretend to be dead)" , "Play it to pretend to be asleep" .)