Discernment of staff originally employed in gallery (5)
I believe the answer is:
taste
'discernment' is the definition.
(having discernment is having good taste)
'staff originally employed in gallery' is the wordplay.
'originally' says to take the initial letters.
'employed in' indicates putting letters inside.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
The initial letter of 'staff' is 's'.
's' put into 'tate' is 'TASTE'.
'of' is the link.
(Other definitions for taste that I've seen before include "Flavour; discernment" , "Nice distinction felt in the mouth" , "Sample by eating or drinking a little" , "style" , "Sensation in the mouth caused by food" .)