Move back in line all at once (8)
I believe the answer is:
together
'once' is the definition.
(thesaurus)
'move back in line all' is the wordplay.
'move' becomes 'go' (going is a kind of moving).
'back' shows that the letters should be reversed in order.
'in' is an insertion indicator.
'line all' becomes 'tether' (I can't justify this - if you can you should believe this answer much more).
'go' backwards is 'og'.
'og' inserted inside 'tether' is 'TOGETHER'.
'at' acts as a link.
Can you help me to learn more?
(Other definitions for together that I've seen before include "without a break?" , "In one another's company" , "Level-headed" , "in league" , "Well-balanced" .)