Greg and Ernie have a look (6)
I believe the answer is:
gander
'a look' is the definition.
(informal term for a look or glance)
'greg and ernie have' is the wordplay.
'have' indicates a hidden word.
'GANDER' can be found hidden inside 'greg and ernie'.
(Other definitions for gander that I've seen before include "Ranged over the bird" , "Male goose; look (colloq.)" , "Goose sauce is good for it" , "Glimpse" , "Male bird - look" .)