Knight’s title is upheld with support of King (3)
I believe the answer is:
sir
'knight's title' is the definition.
(knights are addressed as 'sir')
'is upheld with support of king' is the wordplay.
'upheld' says the letters should be written backwards.
'with support of' means one lot of letters go next to another (some letters go beneath or support others).
'king' becomes 'R' (abbreviation of Latin rex).
'is' in reverse letter order is 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Form of address for a knight" , "... ___ John Major" , "Term of address for a gentleman" , "Title of Kt or Bart" , "Noble title of undesirable kind" .)