Prepare to come into line (5)
I believe the answer is:
dress
I believe this clue is a double definition.
'prepare' is the first definition.
(I know that dress is a more specific form of the action prepare)
'come into line' is the second definition.
(synonyms)
'to' is the link.
(Other definitions for dress that I've seen before include "deck" , "items to wear" , "Clothe; decorate" , "To prepare" , "This woman might slip on" .)