Fruit: happy to consume the third of four pounds! (8)
I believe the answer is:
mulberry
'fruit' is the definition.
(mulberry is a kind of fruit)
'happy to consume the third of four pounds' is the wordplay.
'happy' becomes 'merry'.
'to consume' means one lot of letters goes inside another.
'the third of four' becomes 'u' (3rd letter of 'four').
'pounds' becomes 'lb' (abbreviation for the unit of weight).
'u'+'lb'='ulb'
'merry' enclosing 'ulb' is 'MULBERRY'.
(Other definitions for mulberry that I've seen before include "Tree whose leaves feed silkworms" , "dark purple" , "plant" , "Bush in nursery rhyme" , "Blackberry-like fruit" .)