Flammable gin cocktail on one counter (9)
I believe the answer is:
ignitable
'flammable' is the definition.
(synonyms)
'gin cocktail on one counter' is the wordplay.
'cocktail' indicates an anagram.
'on' is an insertion indicator (as in clothing 'on' a person).
'one' becomes 'i' (Roman numeral).
'counter' becomes 'table' (I've seen this before).
'gin' with letters rearranged gives 'gni'.
'gni' going around 'i' is 'igni'.
'igni'+'table'='IGNITABLE'
(Other definitions for ignitable that I've seen before include "likely to catch fire" , "likely to light up" .)