Day before American artist's retrospective, it's blocked (9)
I believe the answer is:
snookered
'blocked' is the definition.
(I've seen this before)
'day before american artist's retrospective' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'before' becomes 'ere' (poetic term for 'before').
'american artist' becomes 'Koons' (American artist Jeff Koons).
'retrospective' shows that the letters should be reversed in order.
'd'+'ere'+'koons'='derekoons'
'derekoons' in reverse letter order is 'SNOOKERED'.
'it's' acts as a link.
(Other definitions for snookered that I've seen before include "Thwarted" , "Put in difficult position" , "blocked" .)