Peacekeepers reflected obvious sort of power (7)
I believe the answer is:
nuclear
'sort of power' is the definition.
'peacekeepers reflected obvious' is the wordplay.
'peacekeepers' becomes 'UN' (United Nations, peacekeeping organisation).
'reflected' says the letters should be written in reverse (letters reflected back).
'obvious' becomes 'clear' (similar in meaning).
'un' reversed gives 'nu'.
'nu'+'clear'='NUCLEAR'
(Other definitions for nuclear that I've seen before include "An ulcer (anag)" , "Such a bomb" , "and 6 Down: Power producer" , "source of energy" , "power source" .)