Indicate school education regresses (6)
I believe the answer is:
denote
'indicate' is the definition.
(I've seen this before)
'school education regresses' is the wordplay.
'school' becomes 'eton' (famous UK public school).
'education' becomes 'ed' (abbreviation - e.g. sex ed).
'regresses' says the letters should be written backwards (I've seen 'regress' mean this).
'eton'+'ed'='etoned'
'etoned' in reverse letter order is 'DENOTE'.
(Other definitions for denote that I've seen before include "Indicate, signify" , "Designate" , "show" , "Indicate, be a sign of" , "Stand as symbol for" .)