Key volume on much of Mediterranean country (5)
I believe the answer is:
vital
'key' is the definition.
(I know that key can be written as vital)
'volume on much of mediterranean country' is the wordplay.
'volume' becomes 'v' (abbreviation for volume in book titles).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'much of' means to remove the last letter.
'mediterranean country' becomes 'italy' (I've seen this before).
'italy' with its final letter taken away is 'ital'.
'v'+'ital'='VITAL'
(Other definitions for vital that I've seen before include "Imperative" , "Extremely important, essential to success" , "Are such statistics" , "Necessary for life" , "Critical; life-sustaining" .)