Eric's upset about Len knitting a muffler (8)
I believe the answer is:
silencer
'muffler' is the definition.
(I know that muffler can be written as silencer)
'eric's upset about len' is the wordplay.
'upset' is an anagram indicator.
'about' is an insertion indicator.
'erics' is an anagram of 'sicer'.
'sicer' enclosing 'len' is 'SILENCER'.
'knitting a' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for silencer that I've seen before include "It deadens the sound of firing" , "Exhaust quietener" , "Gun noise reducer" , "Fitment for exhaust or gun barrel" , "Device reducing a gun's noise" .)