Glares about junction being most sizeable (7)
I believe the answer is:
largest
'most sizeable' is the definition.
('large' can be similar in meaning to 'sizeable')
'glares about junction' is the wordplay.
'about' indicates an anagram.
'junction' becomes 't'.
'glares' with letters rearranged gives 'larges'.
'larges'+'t'='LARGEST'
'being' acts as a link.
(Other definitions for largest that I've seen before include "Biggest" , "Maximum" , "Most substantial" , "Chief" , "Extremely generous" .)