King's imprimatur lends organisation at first a certain weight (4)
I believe the answer is:
kilo
'a certain weight' is the definition.
(kilogram)
'king's imprimatur lends organisation at first' is the wordplay.
'at first' says to take the initial letters.
The initial letters of 'kings imprimatur lends organisation' is 'KILO'.
(Other definitions for kilo that I've seen before include "possible drugs unit" , "Metric weight, informally" , "Unit of weight (abbr)" , "Metric prefix meaning 1,000" , "Approx 2 point 205 lbs" .)