Glossary requires study once mounted digital display unit missing power leads (7)
I believe the answer is:
lexicon
'glossary' is the definition.
(thesaurus)
'study once mounted digital display unit missing power leads' is the wordplay.
I cannot quite see how this works, but
'study' could be 'con' (con can mean to study) and 'con' is present in the answer.
'once' could be 'ex' (prefix meaning former or onetime) and 'ex' is located in the answer.
'unit' could be 'l' (l is a kind of unit) and 'l' is found in the answer.
A single letter 'i' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'requires' is the link.
Can you help me to learn more?
(Other definitions for lexicon that I've seen before include "Dictionary of terms" , "Dictionary as of Greek or Hebrew" , "Nice lox (anag)" , "Wordbook" , "set of terms" .)