Expanding space - glaring mistake (9)
I believe the answer is:
enlarging
'expanding' is the definition.
(thesaurus)
'space glaring mistake' is the wordplay.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'mistake' indicates anagramming the letters.
'glaring' anagrammed gives 'larging'.
'en'+'larging'='ENLARGING'
(Other definitions for enlarging that I've seen before include "Making bigger" , "speaking at length" .)