Mark's terrified, neglecting education (4)
I believe the answer is:
scar
'mark's' is the definition.
('scar' can be a synonym of 'mark')
'terrified neglecting education' is the wordplay.
'terrified' becomes 'scared'.
'neglecting' suggests deleting specific letters.
'education' becomes 'ed' (abbreviation - e.g. sex ed).
'scared' with 'ed' removed is 'SCAR'.
(Other definitions for scar that I've seen before include "A bare precipice" , "Trace of an old injury" , "Skin blemish - bare crag" , "Mark left on healed tissue" , "Identifying mark" .)