Powerful teacher first is obstinate (10)
I believe the answer is:
headstrong
'obstinate' is the definition.
(I've seen this before)
'powerful teacher first' is the wordplay.
'powerful' becomes 'strong' (associated in meaning).
'teacher' becomes 'head'.
'first' means one lot of letters go next to another.
'strong' put after 'head' is 'HEADSTRONG'.
'is' is the link.
(Other definitions for headstrong that I've seen before include "obstinately self-willed" , "pig-headed" , "Wilfully determined" , "strong-willed" , "Showing wilfulness" .)