Assumed name lacks heart, unfortunately (4)
I believe the answer is:
alas
'unfortunately' is the definition.
('alas' is used to express regret)
'assumed name lacks heart' is the wordplay.
'assumed name' becomes 'alias' (synonyms).
'lacks heart' means to remove the middle letters (I've seen 'lacking heart' mean this).
'alias' with its middle taken out is 'ALAS'.
(Other definitions for alas that I've seen before include "Exclamation of misfortune" , "Cry of woe" , "I'm sorry to say" , "Interjection of sorrow" , "Lackaday" .)