Flesh is old in type of diagram (7)
I believe the answer is:
venison
'flesh' is the definition.
(I have seen 'Flesh of deer' mean 'venison' so perhaps 'flesh' could also mean 'venison')
'is old in type of diagram' is the wordplay.
I cannot really understand how this works, but
'is' is found in the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for venison that I've seen before include "Game" , "Meat - in ovens (anag)" , "Type of meat" , "Flesh of deer" , "Deer meat" .)