Terribly inert host becoming most prickly (9)
I believe the answer is:
thorniest
'most prickly' is the definition.
('thorny' can be a synonym of 'prickly')
'terribly inert host' is the wordplay.
'terribly' indicates anagramming the letters.
'inert'+'host'='inerthost'
'inerthost' with letters rearranged gives 'THORNIEST'.
'becoming' acts as a link.
(Other definitions for thorniest that I've seen before include "most problematic" , "Most troublesome or prickly" .)