Tough peacekeepers, brave (though not at first) (7)
I believe the answer is:
unlucky
'tough' is the definition.
(tough can mean unlucky or unfortunate)
'peacekeepers brave though not at first' is the wordplay.
'peacekeepers' becomes 'UN' (United Nations, peacekeeping organisation).
'brave' becomes 'plucky' (I've seen this before).
'though not at first' means to remove the first letter (the first letter is removed).
'plucky' with its first letter taken off is 'lucky'.
'un'+'lucky'='UNLUCKY'
(Other definitions for unlucky that I've seen before include "Hard cheese!" , "Lacking good fortune" , "Ill-omened" , "Unfortunate" , "Evidently cursed" .)