"Line up!" (3,2,1,4)
'line up' is the definition.
(synonyms)
This is all the clue.
(Other definitions for align that I've seen before include "Straighten in relation to others" , "Regulate, arrange in line" , "Set in order" , "Put in order" , "Arrange so as to be parallel" .)
(Other definitions for one at a time that I've seen before include "Singly" , "A-team, I note, travelling separately" , "Individually" .)