Thing without tail, unknown climbing mountain creature (4)
I believe the answer is:
yeti
'mountain creature' is the definition.
(I've seen this in another clue)
'thing without tail unknown climbing' is the wordplay.
'thing' becomes 'item' (an item is a thing).
'without tail' means to remove the last letter.
'unknown' becomes 'y' (algebra).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'item' with its final letter taken off is 'ite'.
'ite'+'y'='itey'
'itey' backwards is 'YETI'.
(Other definitions for yeti that I've seen before include "Anthropoid cryptid" , "strange creature" , "The Abominable Snowman in Tibetan" , "Mythical Tibetan beast" , "Creature of disputed existence" .)