Shakespeare's return is dull (4)
I believe the answer is:
drab
'is dull' is the definition.
(synonyms)
'shakespeare's return' is the wordplay.
'shakespeare' becomes 'bard' (Shakespeare is known as the Bard).
'return' says the letters should be written backwards.
'bard' back-to-front is 'DRAB'.
(Other definitions for drab that I've seen before include "Dull - dreary" , "Grey or light brown cloth" , "Dull and boring" , "Rather dull" , "Sombre" .)