Material raised seconds after departure (6)
I believe the answer is:
egress
'departure' is the definition.
(I've seen this in another clue)
'material raised seconds' is the wordplay.
'material' becomes 'serge' (serge is a kind of material).
'raised' shows that the letters should be reversed in order (going up in a down clue).
'seconds' becomes 's'.
'serge' in reverse letter order is 'egres'.
'egres'+'s'='EGRESS'
'after' is the link.
(Other definitions for egress that I've seen before include "Way out; going out" , "Departure - exit" , "Issue" , "At the right moment" , "Escape route" .)