Officer is stern over alarm I'd set off (4,7)
I believe the answer is:
rear admiral
'officer' is the definition.
(I know that rear admiral is a type of flag officer)
'stern over alarm i'd set off' is the wordplay.
'stern' becomes 'rear' (synonyms).
'over' is a charade indicator (letters next to each other).
'set off' indicates anagramming the letters.
'alarm'+'id'='alarmid'
'alarmid' is an anagram of 'admiral'.
'rear'+'admiral'='REAR-ADMIRAL'
'is' acts as a link.
(Other definitions for rear admiral that I've seen before include "navy bigwig" , "Top brass" , "Naval officer" , "Naval rank above commodore" .)