Vein right inside sirloin? (6)
I believe the answer is:
streak
'vein' is the definition.
(I've seen this in another clue)
'right inside sirloin?' is the wordplay.
I cannot quite see how this works, but
'right' could be 'r' (common abbreviation) and 'r' is located in the answer.
The remaining letters 'steak' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for streak that I've seen before include "Skater (anag.)" , "irregular line of colour" , "Unbroken series of events" , "run? Barely!" , "Stripe in the hair, say" .)