Blemish of Monica ends at first regret (3)
I believe the answer is:
mar
'blemish' is the definition.
(thesaurus)
'monica ends at first regret' is the wordplay.
'ends' suggests removing the centre.
'at first' indicates taking the first letters.
'monica' with its middle taken out is 'ma'.
The initial letter of 'regret' is 'r'.
'ma'+'r'='MAR'
'of' acts as a link.
(Other definitions for mar that I've seen before include "Spoil something" , "Spanish main?" , "Ram into and spoil" , "Spoil stuff" , "A windy month" .)