Just before midday comes around (4)
I believe the answer is:
mere
'just' is the definition.
(thesaurus)
'before midday comes around' is the wordplay.
'before' becomes 'ere' (usually literary).
'midday' becomes 'm'.
'comes around' says the letters should be written backwards.
'ere'+'m'='erem'
'erem' reversed gives 'MERE'.
(Other definitions for mere that I've seen before include "Pierre's mother identified" , "Unimpressive" , "of no significance" , "Lake; Wiltshire town" , "Nothing more than - water" .)