Best monarch wants small jug (7)
I believe the answer is:
creamer
'small jug' is the definition.
(I've seen this in another clue)
'best monarch' is the wordplay.
'best' becomes 'cream' (as in 'the cream of the crop').
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'cream'+'er'='CREAMER'
'wants' acts as a link.
(Other definitions for creamer that I've seen before include "Coffee whitener" , "Whitening powder" .)