First aperture (7)
I believe the answer is:
opening
This clue is a double definition.
'first' is the first definition.
(similar in meaning)
'aperture' is the second definition.
(I've seen this in another clue)
(Other definitions for opening that I've seen before include "Aperture or opportunity" , "- - time (pub)" , "Beginning, available job" , "launch" , "Rent" .)