Former student struggling in a test is difficult to change (9)
I believe the answer is:
obstinate
'difficult to change' is the definition.
(obstinate means stubborn or unchanging)
'former student struggling in a test' is the wordplay.
'former student' becomes 'OB' (abbreviation for 'Old Boy').
'struggling' indicates an anagram (the letters struggle or wriggle into a new form).
'in'+'a'+'test'='inatest'
'inatest' anagrammed gives 'stinate'.
'ob'+'stinate'='OBSTINATE'
'is' is the link.
(Other definitions for obstinate that I've seen before include "Stubborn or unyielding" , "Inflexible" , "Stubborn, obdurate" , "such as a mule" , "Pig-headed" .)