One third of fruit’s been eaten? Get stuck in (6)
I believe the answer is:
engage
'been eaten? get stuck in' is the definition.
I can't judge whether this defines the answer.
'one third of fruit's' is the wordplay.
'one third of' becomes 'en' (I am not sure about this - if you are sure you should believe this answer much more).
'fruit' becomes 'gage' (a greengage or gage is a type of fruit).
'en'+'gage'='ENGAGE'
Can you help me to learn more?
(Other definitions for engage that I've seen before include "Join" , "Employ or pledge" , "Pay to serve" , "Employ or captivate" , "Hire for work" .)