Always some reversal (4)
I believe the answer is:
ever
'always' is the definition.
(synonyms)
'some reversal' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'EVER' is hidden within 'reversal'.
(Other definitions for ever that I've seen before include "Perpetually" , "Possibly" , "Continuously" , "Always and permanent" , "On any occasion" .)