Ghastly den mostly used by schoolboys originally (7)
I believe the answer is:
hideous
'ghastly' is the definition.
(thesaurus)
'den mostly used by schoolboys originally' is the wordplay.
'den' becomes 'hideout' ('hideout' can be a synonym of 'den').
'mostly' means to remove the last letter.
'used by' means one lot of letters go next to another.
'schoolboys originally' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'hideout' with its last letter taken away is 'hideou'.
'hideou'+'s'='HIDEOUS'
Can you help me to learn more?
(Other definitions for hideous that I've seen before include "Extremely ugly" , "Ghastly" , "revolting" , "Horrible" , "Extremely unpleasant" .)