King departing, crab-like, is unusually crabby (9)
I believe the answer is:
irascible
'crabby' is the definition.
'king departing crab-like is unusually' is the wordplay.
'king' becomes 'k'.
'departing' suggests deleting specific letters.
'unusually' indicates anagramming the letters.
'crab'+'like'+'is'='crablikeis'
'crablikeis' with 'k' taken out is 'crablieis'.
'crablieis' with letters rearranged gives 'IRASCIBLE'.
(Other definitions for irascible that I've seen before include "Cross" , "Quick-tempered" , "Crabbed" , "Easily aroused to anger" , "testy" .)