Girl’s ultimately regretful feeling (5)
I believe the answer is:
laura
'girl's' is the definition.
'ultimately regretful feeling' is the wordplay.
'ultimately' suggests the final letters.
'feeling' becomes 'aura' (both can mean a mood or atmosphere).
The last letter of 'regretful' is 'l'.
'l'+'aura'='LAURA'
(Other definitions for laura that I've seen before include "__ Ashley, Welsh fashion and textile designer, d. 1985" , "girl" , "Girl's name" , "- Ashley, Welsh clothes, furnishings designer" , "- Ashley, Welsh clothing and furniture designer" .)