Threatening university with closure of lending library after vacation (4)
I believe the answer is:
ugly
'threatening' is the definition.
(synonyms)
'university with closure of lending library after vacation' is the wordplay.
'university' becomes 'u' (abbreviation for university).
'with' says to put letters next to each other.
'closure of' says to take the final letters.
'after vacation' suggests removing the centre (the inside letters must vacate).
'library' with its middle taken out is 'ly'.
The final letter of 'lending' is 'g'.
'u'+'g'+'ly'='UGLY'
(Other definitions for ugly that I've seen before include "Revolting" , "describe duckling thus" , "Menacing" , "Grotesque" , "Not pleasing to look at" .)