Acerbic teacher vacated transport (9)
I believe the answer is:
trenchant
'acerbic' is the definition.
(I've seen this before)
'teacher vacated transport' is the wordplay.
'vacated' says to hollow out the word (remove centre letters).
'transport' becomes 'enchant' ('enchant' can be a synonym of 'transport'**).
'teacher' with its middle removed is 'tr'.
'tr'+'enchant'='TRENCHANT'
(Other definitions for trenchant that I've seen before include "Keen" , "Showing keenness" , "Sharp" , "Distinctly defined" , "Acerbic" .)