Leaders of Scots are disappointed and glum (3)
I believe the answer is:
sad
'glum' is the definition.
(thesaurus)
'leaders of scots are disappointed' is the wordplay.
'leaders of' says to take the initial letters.
The initial letters of 'scots are disappointed' is 'SAD'.
'and' is the link.
(Other definitions for sad that I've seen before include "Dismal" , "Melancholy, triste" , "'Unhappy, sorry (3)'" , "Blue" , "Doleful" .)