Regrettably adopting one false identity (5)
I believe the answer is:
alias
'false identity' is the definition.
(I've seen this before)
'regrettably adopting one' is the wordplay.
'regrettably' becomes 'alas' (I've seen this before).
'adopting' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'alas' placed around 'i' is 'ALIAS'.
(Other definitions for alias that I've seen before include "Pseudonym" , "one's assumed" , "Cabbage salad" , "A sail (anag)" , "Also known as" .)