One's continually upset to some extent (4)
I believe the answer is:
unit
'one's' is the definition.
(synonyms)
'continually upset to some extent' is the wordplay.
'to some extent' indicates the answer is hidden within the clue.
'upset' is a reversal indicator.
'tinu' is hidden in the letters of 'continually'.
'tinu' back-to-front is 'UNIT'.
(Other definitions for unit that I've seen before include "Self-contained part" , "One; military group" , "- - trust" , "Mexican maize pancake" , "Body of soldiers" .)