Criticise time taken by BT, getting back on line (6)
I believe the answer is:
berate
'criticise' is the definition.
(berating is a kind of criticizing)
'time taken by bt getting back on line' is the wordplay.
'time' becomes 'era' (an era is a period of time).
'taken by' is an insertion indicator.
'getting' is a charade indicator (letters next to each other).
'back on' indicates one should take the final letters (the letter at the back).
The last letter of 'line' is 'e'.
'era' inserted within 'bt' is 'berat'.
'berat'+'e'='BERATE'
(Other definitions for berate that I've seen before include "Reprimand" , "Rebuke" , "Carpet" , "Censure severely or angrily" , "Condemn" .)