Tears out the last letter; it's only a substitute (6)
I believe the answer is:
ersatz
'a substitute' is the definition.
(I know that substitute can be written as ersatz)
'tears out the last letter' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'the last letter' becomes 'Z' (last letter of the alphabet).
'tears' is an anagram of 'ersat'.
'ersat'+'z'='ERSATZ'
'it's only' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ersatz that I've seen before include "Artificial or synthetic, from German" , "Artificial, imitation - from German" , "Phoney as in Germany" , "Bogus" , "Imitation or substitute, from German" .)