Power on train affected, without a doubt (3,7)
I believe the answer is:
for certain
'without a doubt' is the definition.
(I've seen this before)
'power on train affected' is the wordplay.
'power' becomes 'force' (synonyms).
'on' indicates putting letters inside.
'affected' indicates an anagram.
'train' with letters rearranged gives 'rtain'.
'force' put into 'rtain' is 'FOR CERTAIN'.
(Other definitions for for certain that I've seen before include "definitely" , "without a doubt" , "Assuredly" , "surely" .)