End up cut and beset by an insect (9)
I believe the answer is:
terminate
'end' is the definition.
(I know that end can be written as terminate)
'up cut and beset by an insect' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'cut' means to remove the last letter.
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'an insect' becomes 'termite' (termite is a kind of insect).
'and' with its last letter removed is 'an'.
'an' reversed gives 'na'.
'na' going into 'termite' is 'TERMINATE'.
(Other definitions for terminate that I've seen before include "Put an end to, discontinue" , "Stop" , "Cut short" , "Finalise" , "Kill off" .)