Possible Clues:

• Intractable
• Like a mule turning into beast
• Stubborn old boy's dined, purchasing can
• Wilful child mostly climbing, say, around home
• Intransigent — beat it, son (anag)
• Inflexible manifesto ultimately popular within British government
• Old boy, say, purchasing popular firm
• Former student struggling in a test is difficult to change
• Stubborn old boy, say, outside home
• Pig-headed botanist stupidly taking an E