Answer's to be found in proven and reliable political publication (7)
I believe the answer is:
hansard
'political publication' is the definition.
(record of parliamentary debates)
'answer's to be found in proven and reliable' is the wordplay.
'answer' becomes 'ans' (abbreviation for answer).
'to be found in' indicates putting letters inside.
'proven and reliable' becomes 'hard' (as in hard facts).
'ans' going within 'hard' is 'HANSARD'.
(Other definitions for hansard that I've seen before include "Official record of parliamentary debates" , "political publication" , "Verbatim report of UK parliamentary proceedings" , "UK parliamentary record" , "Official printed record of Parliamentary speeches" .)