Peter Sellers' stuff is brief and pithy (5)
I believe the answer is:
terse
'brief and pithy' is the definition.
(I've seen this in another clue)
'peter sellers' stuff' is the wordplay.
'stuff' says the answer is hidden in the clue (I've seen 'stuffing' mean this).
'TERSE' is hidden within 'peter sellers'.
'is' acts as a link.
(Other definitions for terse that I've seen before include "Curt, concise" , "Curt among trees" , "Pithy and concise" , "Trees (anag.)" , "Concise about trees" .)