Line taken by leaderless crew shows brilliance (5)
I believe the answer is:
light
'brilliance' is the definition.
(brilliant can mean shining or lit up)
'line taken by leaderless crew' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'taken by' means one lot of letters go next to another.
'leaderless' means to remove the first letter.
'crew' becomes 'eight' (an eight-person rowing team).
'eight' with its first letter removed is 'ight'.
'l'+'ight'='LIGHT'
'shows' acts as a link.
(Other definitions for light that I've seen before include "Source of illumination" , "Moored vessel warning sailors of a hazard" , "Weightless; lamp" , "Airy; luminescence" , "guide in the dark" .)