Vein, primarily red, in cut of meat (6)
I believe the answer is:
streak
'vein' is the definition.
(I've seen this in another clue)
'primarily red in cut of meat' is the wordplay.
'primarily red' becomes 'r' (I've seen this in other clues. I am not sure about the 'primarily' bit.).
'in' means one lot of letters goes inside another.
'cut of meat' becomes 'steak' (steak is a kind of cut of meat).
'r' inserted within 'steak' is 'STREAK'.
Can you help me to learn more?
(Other definitions for streak that I've seen before include "series of lucky wins" , "Smear - run naked" , "Naked public appearance" , "Stripe of contrasting colour" , "'Sudden flash, of lightning say (6)'" .)