Note medic starts to investigate blood in vein (6)
I believe the answer is:
midrib
'vein' is the definition.
(I've seen this before)
'note medic starts to investigate blood' is the wordplay.
'note' becomes 'mi' (as in do-re-mi).
'medic' becomes 'dr' (doctor).
'starts to' indicates taking the first letters.
The first letters of 'investigate blood' is 'ib'.
'mi'+'dr'+'ib'='MIDRIB'
'in' acts as a link.
(Other definitions for midrib that I've seen before include "Part of leaflet" , "Large vein on leaf" , "Central leaf vein" , "major vein" , "part of leaf" .)