Unknown garment hung up in the old cell (6)
I believe the answer is:
zygote
'cell' is the definition.
(zygote is a kind of cell)
'unknown garment hung up in the old' is the wordplay.
'unknown' becomes 'z' (algebra).
'garment' becomes 'tog' ('tog' can be a synonym of 'garment').
'hung up' says the letters should be written backwards.
'in' is an insertion indicator.
'the old' becomes 'ye'.
'tog' in reverse letter order is 'got'.
'got' put within 'ye' is 'ygote'.
'z'+'ygote'='ZYGOTE'
(Other definitions for zygote that I've seen before include "Cell resulting from two gametes fusing" , "A fertilised ovum" , "Egg/sperm-union cell" .)