Formerly connected with every other member’s disappearance (4)
I believe the answer is:
once
'formerly' is the definition.
(I know that formerly can be written as once)
'connected with every other member's disappearance' is the wordplay.
'with every other member's disappearance' means one should take alternating letters (every other letter is removed).
The alternate letters of 'connected' are 'ONCE'.
(Other definitions for once that I've seen before include "On one occasion" , "A single time" , "In former times" , "Formerly, at one time" , "Quick examination" .)