Boleyn, Henry’s second to go, treated not in this way! (5)
I believe the answer is:
nobly
'not in this way' is the definition.
The definition and answer are not the same part of speech.
'boleyn henry's second to go treated' is the wordplay.
'henry's second' becomes 'e' (2nd letter of 'henry').
'to go' indicates named letters should be taken away.
'treated' indicates an anagram.
'boleyn' with 'e' taken out is 'bolyn'.
'bolyn' is an anagram of 'NOBLY'.
(Other definitions for nobly that I've seen before include "With great dignity" , "Worthily" , "In an aristocratic manner" .)