Summons primarily will restrain its target (4)
I believe the answer is:
writ
'summons' is the definition.
(thesaurus)
'primarily will restrain its target' is the wordplay.
'primarily' says to take the initial letters.
The first letters of 'will restrain its target' is 'WRIT'.
(Other definitions for writ that I've seen before include "Legal or formal document" , "Sealed document" , "Formal command of law court requiring certain action" , "Legal injunction" , "Legal document (usually issued)" .)