Discernment of gallery holding Sisley opening (5)
I believe the answer is:
taste
'discernment' is the definition.
('taste' can be a synonym of 'discernment')
'gallery holding sisley opening' is the wordplay.
'gallery' becomes 'tate' (I've seen this before).
'holding' means one lot of letters goes inside another.
'opening' says to take the initial letters.
The initial letter of 'sisley' is 's'.
'tate' enclosing 's' is 'TASTE'.
'of' acts as a link.
(Other definitions for taste that I've seen before include "Judgment" , "Impression from food" , "Sample by eating or drinking a little" , "Smack" , "Appreciation, discernment" .)