Worker on newspaper once reviewing poor sitcom (10)
I believe the answer is:
compositor
'worker on newspaper' is the definition.
(someone who arranges type for printing)
'reviewing poor sitcom' is the wordplay.
'reviewing' indicates an anagram (To re view).
'poor'+'sitcom'='poorsitcom'
'poorsitcom' anagrammed gives 'COMPOSITOR'.
'once' is the link.
(Other definitions for compositor that I've seen before include "typesetter" , "One assembling material" , "Print worker" , "I set in print room" , "one in the printing industry" .)