Striking red songbird in another's nest to start with (6)
I believe the answer is:
titian
'striking red' is the definition.
(titian can mean a red colour)
'songbird in another's nest to start with' is the wordplay.
'songbird' becomes 'tit' (I've seen this before).
'to start with' says to take the initial letters.
The initial letters of 'in anothers nest' is 'ian'.
'tit'+'ian'='TITIAN'
(Other definitions for titian that I've seen before include "artist from Venice" , "Italian painter of the Venetian school, patronised by Philip II of Spain" , "Italian Old Master and colour" , "It. painter, famous for redheads" , "Reddish-gold; painter" .)