Flares set off by officer in the open (8)
I believe the answer is:
alfresco
'in the open' is the definition.
(thesaurus)
'flares set off by officer' is the wordplay.
'set off' is an anagram indicator.
'by' means one lot of letters go next to another.
'officer' becomes 'co' (Commanding Officer abbreviation).
'flares' is an anagram of 'alfres'.
'alfres'+'co'='ALFRESCO'
(Other definitions for alfresco that I've seen before include "exposed to the fresh air" , "Outside" , "where it's cool" , "Outdoors, meal perhaps" , "Meal eaten in the outdoors" .)