Somewhat united in protection of King Henry as usual (8)
I believe the answer is:
habitual
'usual' is the definition.
(thesaurus)
'somewhat united in protection of king henry' is the wordplay.
'somewhat' becomes 'abit' (a bit).
'united' becomes 'u'.
'in protection of' is an insertion indicator.
'king henry' becomes 'hal' (nickname of Henry V in Shakespeare's plays).
'abit'+'u'='abitu'
'abitu' inserted into 'hal' is 'HABITUAL'.
'as' acts as a link.
(Other definitions for habitual that I've seen before include "Commonly practised" , "Traditional" , "Done constantly or regularly (8)" , "Like a routine" , "Common" .)