Extremely fine cutting for instance is all that matters (10)
I believe the answer is:
everything
'all that matters' is the definition.
'everything' can be an answer for 'all' (all things). I'm not certain of the remainder of the definition.
'extremely fine cutting for instance' is the wordplay.
'extremely' becomes 'very' (very can mean extremely or considerably).
'fine' becomes 'thin' (similar in meaning).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'for instance' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'very'+'thin'='verythin'
'verythin' put within 'eg' is 'EVERYTHING'.
'is' is the link.
Can you help me to learn more?
(Other definitions for everything that I've seen before include "The whole lot" , "great deal" , "All items" , "The whole shebang" , "the works" .)