Expert about to break best form of digital security (6)
I believe the answer is:
toecap
'form of digital security' is the definition.
'expert about to break best' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert**).
'about' says the letters should be written in reverse.
'to break' indicates putting letters inside.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'ace' backwards is 'eca'.
'eca' placed inside 'top' is 'TOECAP'.
(Other definitions for toecap that I've seen before include "Reinforced part of shoe or boot" , "Tip of shoe" , "Protective cover on boot or shoe" , "Steel part for shoe" , "Boot's metal end" .)