Old, old soldier protecting King? That's obvious (5)
I believe the answer is:
overt
'obvious' is the definition.
('overt' can be similar in meaning to 'obvious')
'old old soldier protecting king?' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'old soldier' becomes 'vet' (I've seen this before).
'protecting' is an insertion indicator.
'king?' becomes 'r' (r is abbreviation for rex).
'vet' enclosing 'r' is 'vert'.
'o'+'vert'='OVERT'
'that's' is the link.
(Other definitions for overt that I've seen before include "Conspicuous" , "Not locked away" , "____ is able to be seen" , "Open and observable" , "Open and obvious" .)