One metal, uranium, is withdrawn (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'metal uranium is withdrawn' is the wordplay.
'metal' becomes 'tin' (tin is a kind of metal).
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'is withdrawn' is a reversal indicator.
'tin'+'u'='tinu'
'tinu' written backwards gives 'UNIT'.
(Other definitions for unit that I've seen before include "Individual item" , "Self-contained part" , "Quantity chosen as standard, or individual thing" , "Kitchen element?" , "A considerable number of soldiers" .)