Viewers last in queue? Certainly! (4)
I believe the answer is:
eyes
'viewers' is the definition.
(I've seen this in another clue)
'last in queue? certainly' is the wordplay.
'last in' indicates one should take the final letters.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
The final letter of 'queue' is 'e'.
'e'+'yes'='EYES'
(Other definitions for eyes that I've seen before include "Spots on a peacock's tail" , "Marks on potatoes" , "Witnesses" , "Regards -- features of a potato" , "Narrowly watches" .)