Lovely working out (8)
I believe the answer is:
cracking
I believe this clue is a double definition.
'lovely' is the first definition.
(cracking can informally mean lovely or brilliant)
'working out' is the second definition.
(crack can mean to decipher or work out)
(Other definitions for cracking that I've seen before include "Solving" , "great!" , "failing" , "very fast" , "Wonderful" .)