Not upset, ambassador and leftist in a row (2,3,4)
I believe the answer is:
on the trot
'a row' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'not upset ambassador and leftist' is the wordplay.
'upset' indicates anagramming the letters.
'ambassador' becomes 'HE' (His or Her Excellency).
'and' means one lot of letters go next to another.
'leftist' becomes 'trot' (I can't justify this - if you can you should give a lot more credence to this answer).
'not' with letters rearranged gives 'ont'.
'ont'+'he'+'trot'='ON THE TROT'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for on the trot that I've seen before include "in a row?" , "Continually busy" , "Where orthodox Stalinist hurled anathemas" , "without a break" , "One after another" .)