Open wide, in big aperture (4)
I believe the answer is:
gape
'open wide' is the definition.
(I've seen this before)
'in big aperture' is the wordplay.
'in' indicates a hidden word.
'GAPE' is hidden in the letters of 'big aperture'.
(Other definitions for gape that I've seen before include "Appear amazed" , "Look amazed" , "Look at with wide-open mouth" , "Stare at page" , "Hang wide open" .)