Spike's line brilliant (4)
I believe the answer is:
lace
'spike's' is the definition.
(synonyms)
'line brilliant' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'brilliant' becomes 'ace' (ace can informally mean very good).
'l'+'ace'='LACE'
(Other definitions for lace that I've seen before include "Famous play ''Arsenic and Old . . . .''" , "Fine needlework" , "Fine decorative fabric" , "Fabric made in openwork design" , "Spike - ornamental fabric" .)