Rotten, almost entirely inedible meat (5)
I believe the answer is:
offal
'inedible meat' is the definition.
'offal' can be an answer for 'meat' (offal is a kind of meat). I am unsure of the 'inedible' bit.
'rotten almost entirely' is the wordplay.
'rotten' becomes 'off' (rotten food is 'off').
'almost' means to remove the last letter (most of the word but not all of it).
'entirely' becomes 'all' (all can mean entirely or completely).
'all' with its final letter taken away is 'al'.
'off'+'al'='OFFAL'
Can you help me to learn more?
(Other definitions for offal that I've seen before include "Rejected parts of a carcase" , "Tripe, sweetbreads" , "Animal entrails and internal organs as rubbish" , "Internal organs of an animal used as food" , "Butcher's stock" .)