Tail of aircraft worked very nicely, shortening by 50% around Mach 1 (9)
I believe the answer is:
transonic
'around mach 1' is the definition.
(I've seen this before)
'tail of aircraft worked very' is the wordplay.
'tail of' suggests the final letters.
'worked' becomes 'ran' (running is a kind of working).
'very' becomes 'sonic' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The last letter of 'aircraft' is 't'.
't'+'ran'+'sonic'='TRANSONIC'
'nicely shortening by 50' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for transonic that I've seen before include "Close to speed of sound" , "around Mach 1" .)