Obscure cuttings found in empty envelope (7)
I believe the answer is:
eclipse
'obscure' is the definition.
(eclipsing is a kind of obscuring)
'cuttings found in empty envelope' is the wordplay.
'cuttings' becomes 'clips' (clip is a kind of cutting).
'found in' indicates putting letters inside.
'empty' suggests removing the centre.
'envelope' with its centre taken out is 'ee'.
'clips' going into 'ee' is 'ECLIPSE'.
(Other definitions for eclipse that I've seen before include "Heavenly event; drake's time of dull plumage" , "Outdo" , "Surpass in achievement" , "concealment" , "Throw into shade, like a heavenly body" .)