Agents taking test recalled type of power (6)
I believe the answer is:
atomic
'type of power' is the definition.
'agents taking test recalled' is the wordplay.
'agents' becomes 'CIA' (Central Intelligence Agency).
'taking' is an insertion indicator.
'test' becomes 'MOT' (MOT test).
'recalled' says the letters should be written in reverse (to recall can mean to bring something back).
'cia' going around 'mot' is 'cimota'.
'cimota' reversed gives 'ATOMIC'.
(Other definitions for atomic that I've seen before include "Sort of bomb, number, or power" , "Based on nuclear energy" , "I'm Cato (anag.)" , "nuclear issues?" , "Sort of energy or number" .)