Outstanding peacekeepers put down roots (9)
I believe the answer is:
unsettled
'outstanding' is the definition.
(I've seen this before)
'peacekeepers put down roots' is the wordplay.
'peacekeepers' becomes 'un'.
'put' becomes 'settle' (settling is a kind of putting).
'roots' indicates taking the first letters (I've seen 'root of' mean this).
The first letter of 'down' is 'd'.
'un'+'settle'+'d'='UNSETTLED'
(Other definitions for unsettled that I've seen before include "Not sure" , "a forecast of "showery"?" , "Liable to change or upset" , "Still to confirm" , "Devoid of colonists" .)