Brilliant old American lineage (12)
I believe the answer is:
incandescent
'brilliant' is the definition.
(I've seen this before)
'old american lineage' is the wordplay.
'old american' becomes 'incan' (I've seen this before).
'lineage' becomes 'descent' (descent is a kind of lineage).
'incan'+'descent'='INCANDESCENT'
(Other definitions for incandescent that I've seen before include "White-hot" , "Glowing hot" , "Brilliant" , "livid" , "extremely angry" .)