One can turn upside-down (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'can turn upside-down' is the wordplay.
'can' becomes 'tin' (both are metal containers).
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'upside down' shows that the letters should be reversed in order.
'tin'+'u'='tinu'
'tinu' backwards is 'UNIT'.
(Other definitions for unit that I've seen before include "Operational team" , "One part of a whole" , "Self-contained part" , "Kitchen fitting" , "Item combining separate parts" .)