Take off and fly — go off on sortie originally (9)
I believe the answer is:
skedaddle
'take off' is the definition.
(I've seen this in another clue)
'fly go off on sortie originally' is the wordplay.
'fly' becomes 'ked' (type of parasitic fly).
'go off' becomes 'addle' (eg an egg might addle).
'on' says to put letters next to each other.
'originally' says to take the initial letters.
The initial letter of 'sortie' is 's'.
'ked'+'addle'='kedaddle'
'kedaddle' after 's' is 'SKEDADDLE'.
'and' acts as a link.
(Other definitions for skedaddle that I've seen before include "Depart quickly (colloq.)" , "Hightail it" , "Run off hurriedly (colloq.)" , "Get out of town!" , "Scurry off" .)