Wise to protect head of state after defence department's upgrade (9)
I believe the answer is:
modernise
'upgrade' is the definition.
'wise to protect head of state after defence department's' is the wordplay.
'wise' becomes 'ernie' (Ernie Wise).
'to protect' is an insertion indicator.
'head of' says to take the initial letters.
'after' means one lot of letters go next to another.
'defence department' becomes 'mod' (Ministry of Defence).
The initial letter of 'state' is 's'.
'ernie' placed around 's' is 'ernise'.
'ernise' after 'mod' is 'MODERNISE'.
(Other definitions for modernise that I've seen before include "to adapt to today's world" , "Make up to date" .)