Sharpness was effective cutting very fine line (8)
I believe the answer is:
acerbity
'sharpness' is the definition.
(thesaurus)
'was effective cutting very fine line' is the wordplay.
'was effective' becomes 'bit' (I've seen this in other clues).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'very fine' becomes 'ace' (ace can mean excellent).
'line' becomes 'ry' (abbreviation for railway).
'ace'+'ry'='acery'
'bit' put into 'acery' is 'ACERBITY'.
(Other definitions for acerbity that I've seen before include "Sharp/forthright manner" , "Sourness" , "Harshness or sharpness of speech" , "Sharpness, forthrightness" , "Bitterness, sharpness" .)