Shielding light from poster in exhibition (9)
I believe the answer is:
shadowing
'shielding light' is the definition.
'poster in exhibition' is the wordplay.
'poster' becomes 'ad'.
'in' indicates putting letters inside.
'exhibition' becomes 'showing'.
'ad' inserted into 'showing' is 'SHADOWING'.
'from' is the link.
(Other definitions for shadowing that I've seen before include "Tailing" , "Trailing" .)