Country dinner rather long mostly to prepare (8,7)
I believe the answer is:
northern ireland
'country' is the definition.
(Northern Ireland is an example)
'dinner rather long mostly to prepare' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'to prepare' indicates anagramming the letters (I've seen 'preparing' mean this (the same letters prepared differently)).
'dinner'+'rather'+'long'='dinnerratherlong'
'dinnerratherlong' with its last letter taken away is 'dinnerratherlon'.
'dinnerratherlon' anagrammed gives 'NORTHERN IRELAND'.
(Another definition for northern ireland that I've seen is " Part of the UK".)