A plum diet varies abundance (9)
I believe the answer is:
amplitude
'abundance' is the definition.
(I know that amplitude is a type of abundance)
'a plum diet varies' is the wordplay.
'varies' indicates an anagram.
'plum'+'diet'='plumdiet'
'plumdiet' anagrammed gives 'mplitude'.
'a'+'mplitude'='AMPLITUDE'
(Other definitions for amplitude that I've seen before include "Maximum extent of signal or vibration" , "Abundance, largeness" , "stretch" , "Range" , "Profusion" .)