Marked either end of envelope in a rush (8)
I believe the answer is:
stampede
'rush' is the definition.
(I've seen this in another clue)
'marked either end of envelope in a' is the wordplay.
I cannot really see how this works, but
'a' could be 'amp' (A is the symbol for the amp or ampere) and 'amp' is found within the answer.
The remaining letters 'stede' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for stampede that I've seen before include "Neat rush" , "unstable flight" , "Disorderly rush" , "Headlong rush of a group in panic" , "Deems apt to have flight of cattle" .)