That is upsetting about BBC boss — untouchable but showing nerves (8)
I believe the answer is:
edginess
'nerves' is the definition.
(I've seen this in another clue)
'that is upsetting about bbc boss untouchable' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'upsetting' is a reversal indicator.
'about' is an insertion indicator.
'bbc boss' becomes 'DG' (short for director-general of the BBC).
'untouchable' becomes 'Ness' (Eliot Ness, 'Untouchable' in prohibition era in US).
'ie' reversed gives 'ei'.
'ei' going around 'dg' is 'edgi'.
'edgi'+'ness'='EDGINESS'
'but showing' is the link.
(Other definitions for edginess that I've seen before include "State of nervous tension" , "Feeling of anxiety, disquietude" , "Tenseness, nervousness" .)