Rancour working in support of royal capacity (8,6)
I believe the answer is:
imperial gallon
'capacity' is the definition.
(imperial gallon is a unit of capacity)
'rancour working in support of royal' is the wordplay.
'rancour' becomes 'gall' (gall can mean rancour or resentment).
'working' becomes 'on' (on can mean working or activated).
'in support of' says to put letters next to each other (in a down clue, some letters hold up others).
'royal' becomes 'imperial' (synonyms).
'gall'+'on'='gallon'
'gallon' after 'imperial' is 'IMPERIAL GALLON'.
(Another definition for imperial gallon that I've seen is " 4.55 litres".)