Bill has trouble eating very old fruit (7)
I believe the answer is:
avocado
'fruit' is the definition.
(avocado is a kind of fruit)
'bill has trouble eating very old' is the wordplay.
'bill' becomes 'ac' (short for account).
'has' says to put letters next to each other.
'trouble' becomes 'ado' (both can mean bother or commotion).
'eating' indicates putting letters inside.
'very old' becomes 'vo' (this could be a standard abbreviation of which I'm unaware).
'ac'+'ado'='acado'
'acado' enclosing 'vo' is 'AVOCADO'.
Can you help me to learn more?
(Other definitions for avocado that I've seen before include "Pear-shaped tropical fruit" , "Dark green pear-shaped fruit" , "Dark green fruit like posh pear" , "Light green; fruit" , "may be stoned" .)