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' is a reversal indicator (letters reflected back).
'obvious' becomes 'clear' (similar in meaning).
'un' back-to-front is '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" .)