A delight, the old being entertained by editor quick to notice things (5-4)
I believe the answer is:
eagle-eyed
'quick to notice things' is the definition.
'a delight the old being entertained by editor' is the wordplay.
'delight' becomes 'glee' (I've seen this before).
'the old' becomes 'ye'.
'being entertained by' is an insertion indicator (I've seen this in other clues).
'editor' becomes 'ed'.
'a'+'glee'+'ye'='agleeye'
'agleeye' put inside 'ed' is 'EAGLE-EYED'.
(Other definitions for eagle-eyed that I've seen before include "Very watchful with keen vision" , "Having acute vision" , "Very perceptive" , "Keen-sighted" , "Keenly-sighted" .)