One turns up in continuity (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'turns up in continuity' is the wordplay.
'turns up' is a reversal indicator.
'in' indicates the answer is hidden within the clue.
'tinu' can be found hidden inside 'continuity'.
'tinu' written backwards gives 'UNIT'.
(Other definitions for unit that I've seen before include "Single item, maybe part of a set" , "Constituent" , "Standard for measuring" , "Mexican maize pancake" , "army group" .)