Grumpy king left becoming rebellious (8)
I believe the answer is:
factious
'rebellious' is the definition.
(thesaurus)
'grumpy king left' is the wordplay.
'grumpy' becomes 'fractious' (I've seen this before).
'king' becomes 'r' (r is abbreviation for rex).
'left' is a deletion indicator.
'fractious' with 'r' removed is 'FACTIOUS'.
'becoming' acts as a link.
(Other definitions for factious that I've seen before include "Quarrelsome" , "Ready to quarrel" , "Characterised by internal strife" , "Seditious, divisive" .)