Assembled then to go off, one after the other (2,3,4)
I believe the answer is:
on the trot
'one after the other' is the definition.
(I've seen this before)
'assembled then to go off' is the wordplay.
'assembled' is an anagram indicator.
'go off' becomes 'rot' (I've seen this before).
'then'+'to'='thento'
'thento' anagrammed gives 'onthet'.
'onthet'+'rot'='ON THE TROT'
(Other definitions for on the trot that I've seen before include "Informally busy like slow run" , "One after another" , "without a break" , "Where orthodox Stalinist hurled anathemas" , "In succession" .)