Keep from harm in brilliant finish (6)
I believe the answer is:
defend
'keep from harm' is the definition.
'brilliant finish' is the wordplay.
'brilliant' becomes 'def' (I've seen this before).
'finish' becomes 'end' ('end' can be a synonym of 'finish').
'def'+'end'='DEFEND'
'in' acts as a link.
(Other definitions for defend that I've seen before include "Protect - guard" , "Maintain against attack" , "Protect, fight back" , "Compete to retain (sporting title)" , "Stand up for" .)