Ghastly hat hiding university rector's head (5)
I believe the answer is:
lurid
'ghastly' is the definition.
('lurid' can be a synonym of 'ghastly')
'hat hiding university rector's head' is the wordplay.
'hat' becomes 'lid' ('lid' can be a synonym of 'hat').
'hiding' means one lot of letters goes inside another.
'head' says to take the initial letters.
The first letters of 'university rector' is 'ur'.
'lid' enclosing 'ur' is 'LURID'.
(Other definitions for lurid that I've seen before include "Grim" , "Gloomily threatening" , "Shocking, sensational" , "In shocking detail" , "In glaring colours, shockingly vivid" .)