Energy contained in the majority of rich food (4)
I believe the answer is:
fuel
'energy' is the definition.
(I've seen this before)
'contained in the majority of rich food' is the wordplay.
'contained in' indicates putting letters inside.
'the majority of' means to remove the last letter (most but not all of the word is used).
'rich' becomes 'full' (both can mean bountiful or abundant).
'food' becomes 'e' (E is an example).
'full' with its last letter taken away is 'ful'.
'ful' placed around 'e' is 'FUEL'.
(Other definitions for fuel that I've seen before include "Burned source of energy" , "coal etc" , "Combustible material" , "Substance burned to source power" , "Material fed to flames" .)