Buck up king with bitter underneath throne (4,5)
I believe the answer is:
look sharp
'buck up' is the definition.
'king with bitter underneath throne' is the wordplay.
'king' becomes 'k'.
'with' says to put letters next to each other.
'bitter' becomes 'sharp' (I've seen this before**).
'underneath' means one lot of letters go next to another (I've seen this in other clues).
'throne' becomes 'loo' (both can mean a toilet).
'k'+'sharp' is 'ksharp'.
'ksharp' after 'loo' is 'LOOK SHARP'.
(Other definitions for look sharp that I've seen before include "step on it" , "Be quick!" , "Get cracking" , "keen" , "Hurry up!" .)