Glare could be considerable (5)
I believe the answer is:
large
'considerable' is the definition.
(I've seen this before)
'glare could be' is the wordplay.
'could be' indicates anagramming the letters.
'glare' with letters rearranged gives 'LARGE'.
(Other definitions for large that I've seen before include "Immense" , "Of great size" , "Sizeable" , "Massive" , "Wide-ranging" .)