Brittle BBC boss in first of exacting years (4)
I believe the answer is:
edgy
'brittle' is the definition.
(both can mean nervous)
'bbc boss in first of exacting years' is the wordplay.
'bbc boss' becomes 'DG' (short for director-general of the BBC).
'in' indicates putting letters inside.
'first of exacting' becomes 'e' (1st letter of 'exacting').
'years' becomes 'y' (abbreviation).
'e'+'y'='ey'
'dg' put within 'ey' is 'EDGY'.
(Other definitions for edgy that I've seen before include "Visibly nervous" , "Nervous and tense, irritable" , "Uneasy" , "Apprehensive" , "Extremely" .)