Marker underlining first of howlers in school (6)
I believe the answer is:
harrow
'school' is the definition.
(famous UK public school)
'marker underlining first of howlers' is the wordplay.
'marker' becomes 'arrow' (I've seen this before).
'underlining' means one lot of letters go next to another (in a down clue, some letters go underneath others).
'first of howlers' becomes 'h' (1st letter of 'howlers').
'arrow' after 'h' is 'HARROW'.
'in' is the link.
(Other definitions for harrow that I've seen before include "Soil-breaking implement" , "Cultivator that pulverizes or smooths soil" , "Break up (ground) - distress (feelings)" , "It breaks up clods and covers seeds" , "Equipment provided for ground-breaking" .)