Instruction must be concise (5)
I believe the answer is:
brief
I believe this is a double definition.
'instruction' is the first definition.
(thesaurus)
'concise' is the second definition.
(similar in meaning)
'must be' acts as a link.
(Other definitions for brief that I've seen before include "Short in duration" , "Succinct" , "put in the picture" , "Remit" , "brusque > QC, perhaps" .)