Choose to back, seizing power (3)
I believe the answer is:
opt
'choose' is the definition.
(thesaurus)
'to back seizing power' is the wordplay.
'back' says the letters should be written backwards.
'seizing' indicates putting letters inside.
'power' becomes 'p'.
'to' written backwards gives 'ot'.
'ot' placed around 'p' is 'OPT'.
(Other definitions for opt that I've seen before include "Pick from pot" , "Decide between possibilities, choose" , "End or act of shutting" , "Have preference for something" , "Hold in the highest esteem" .)