Unfortunately shut, a sure store of knowledge (9)
I believe the answer is:
thesaurus
'store of knowledge' is the definition.
(type of reference work)
'unfortunately shut a sure' is the wordplay.
'unfortunately' indicates anagramming the letters (a bad or unfortunate spelling).
'shut'+'a'+'sure'='shutasure'
'shutasure' anagrammed gives 'THESAURUS'.
(Other definitions for thesaurus that I've seen before include "lists?" , "treasury" , "Reference book" , "Word-finder" , "Wordy gathering" .)