Lovely girl briefly going after blackguard, immature creature (6)
I believe the answer is:
caddis
'immature creature' is the definition.
(a caddis is a fly larva)
'lovely girl briefly going after blackguard' is the wordplay.
'lovely girl' becomes 'dish' (term for an attractive person).
'briefly' means to remove the last letter.
'going after' is a charade indicator (letters next to each other).
'blackguard' becomes 'cad' (synonyms).
'dish' with its final letter taken away is 'dis'.
'dis' put after 'cad' is 'CADDIS'.
(Other definitions for caddis that I've seen before include "Mothlike insect's larva" , "Aquatic larva of fly" , "Fly of ponds and streams that camouflages itself" , "worm" , "Immature swimmer" .)