Glare becoming considerable (5)
I believe the answer is:
large
'considerable' is the definition.
(I've seen this before)
'glare becoming' is the wordplay.
'becoming' is an anagram indicator.
'glare' is an anagram of 'LARGE'.
(Other definitions for large that I've seen before include "jumbo" , "Big < burly" , "substantial" , "Big, outsize" , "Of considerable size or extent" .)