Highly skilled teacher likely to be disheartened (8)
I believe the answer is:
masterly
'highly skilled' is the definition.
(I've seen this before)
'teacher likely to be disheartened' is the wordplay.
'teacher' becomes 'master' (I've seen this before**).
'to be disheartened' means to remove the middle letters.
'likely' with its middle taken out is 'ly'.
'master'+'ly'='MASTERLY'
(Other definitions for masterly that I've seen before include "Virtuoso, consummate" , "World-class" , "Supremely skilled" , "Highly skilled" , "Showing exceptional skill" .)