Though yellow at heart it won't run (4-6,3)
I believe the answer is:
hard-boiled egg
'though yellow at heart' is the definition.
The answer and definition can be both things that one consumes for nourishment as well as being singular nouns.
Maybe there's an association between them I don't understand?
'it won't run' is the wordplay.
I cannot really understand how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
'run' could be 'r' (cricket abbreviation) and 'r' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for hard-boiled egg that I've seen before include "Possible picnic item" , "Potential picnic fare" , "Cooked item provided by 1 down" .)