One can turn up (4)
I believe the answer is:
unit
'one' is the definition.
('unit' can be a synonym of 'one')
'can turn up' is the wordplay.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'up' is a reversal indicator (in down clue: letters go upwards).
'tin'+'u'='tinu'
'tinu' in reverse letter order is 'UNIT'.
(Other definitions for unit that I've seen before include "Single undivided entity" , "Piece of furniture" , "Some fighting men" , "Kitchen element?" , "Body of soldiers" .)