Volatile old Conservative on one list (9)
I believe the answer is:
excitable
'volatile' is the definition.
(I've seen this before)
'old conservative on one list' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'conservative' becomes 'C' (UK political abbreviation).
'on' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'list' becomes 'table' (list can mean a table or chart).
'ex'+'c'+'i'+'table'='EXCITABLE'
(Other definitions for excitable that I've seen before include "Easily stimulated" , "Volatile" , "Easily aroused or made emotional" , "Highly strung - easily roused" , "Hot-headed" .)