Know-all's telegram about cracked case (8)
I believe the answer is:
wiseacre
'know-all's' is the definition.
(I've seen this before)
'telegram about cracked case' is the wordplay.
'telegram' becomes 'wire' ('wire' can be a synonym of 'telegram').
'about' indicates putting letters inside.
'cracked' is an anagram indicator.
'case' anagrammed gives 'seac'.
'wire' enclosing 'seac' is 'WISEACRE'.
(Other definitions for wiseacre that I've seen before include "Self-appointed sage" , "Smart alec" , "Person with an affectation of wisdom" , "One affecting superior knowledge" , "Smart Aleck" .)