Stare at a grand primate (4)
I believe the answer is:
gape
'stare at' is the definition.
(I've seen this in another clue)
'a grand primate' is the wordplay.
'a grand' becomes 'g' (G is short for grand as in a thousand).
'primate' becomes 'ape' (large primate).
'g'+'ape'='GAPE'
(Other definitions for gape that I've seen before include "Look in amazement or stupidity" , "Hang wide open" , "Gawp" , "Failure to meet" , "Look amazed" .)