Streak narcissistic, by the sound of it? (4)
I believe the answer is:
vein
'streak' is the definition.
(I've seen this in another clue)
'narcissistic by the sound of it?' is the wordplay.
'narcissistic' becomes 'vain' (I've seen this in another clue).
'by the sound of it?' indicates a 'sounds like' (homophone) clue.
'vain' is a homophone of 'VEIN'.
(Other definitions for vein that I've seen before include "part of body" , "Rock fissure or minerals therein" , "Vessel for claret" , "Distinctive style" , "Irregular streak of colour" .)