Sticking-up hair that's unpleasant under hood (7)
I believe the answer is:
cowlick
'sticking-up hair' is the definition.
I can't tell whether this defines the answer.
'unpleasant under hood' is the wordplay.
'unpleasant' becomes 'ick' (I am not sure about this - if you are sure you should believe this answer much more).
'under' says to put letters next to each other (in a down clue, some letters go below others).
'hood' becomes 'cowl' (synonyms).
'ick' after 'cowl' is 'COWLICK'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for cowlick that I've seen before include "Lock of hair hanging over the forehead" , "shock seen on forehead" , "Tuft of hair that won't lie flat" .)