Smart Aleck's main column's beginning in Telegraph (8)
I believe the answer is:
wiseacre
'smart aleck's' is the definition.
(I've seen this before)
'main column's beginning in telegraph' is the wordplay.
'main' becomes 'sea' ('sea' can be a synonym of 'main').
'beginning' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'telegraph' becomes 'wire' ('wire' can be a synonym of 'telegraph').
The first letter of 'columns' is 'c'.
'sea'+'c'='seac'
'seac' going inside 'wire' is 'WISEACRE'.
(Other definitions for wiseacre that I've seen before include "One claims to know all" , "Smart alec" , "One assuming air of superior wisdom - simpleton" , "Know-it-all" , "Person with an affectation of wisdom" .)