Bad blood as M1 stops Tyne flowing (6)
I believe the answer is:
enmity
'bad blood' is the definition.
(I've seen this before)
'm1 stops tyne flowing' is the wordplay.
'm1' becomes 'mi' (resembles M1).
'stops' indicates putting letters inside.
'flowing' is an anagram indicator.
'tyne' is an anagram of 'enty'.
'mi' placed into 'enty' is 'ENMITY'.
'as' is the link.
(Other definitions for enmity that I've seen before include "Personal hostility" , "Hostility, ill-will" , "evident in time?" , "Animus" , "needle" .)