Conservative policy is heralded by the commencement of secret plot (9)
I believe the answer is:
storyline
'commencement of secret plot' is the definition.
'storyline' can be an answer for 'plot' (storyline is a kind of plot). I'm not sure about the 'commencement of secret' bit.
'conservative policy is heralded by' is the wordplay.
'conservative' becomes 'tory' (informal term for a Conservative Party member).
'policy' becomes 'line' (I've seen this in another clue).
'is heralded' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'by' means one lot of letters go next to another.
'tory'+'line'='toryline'
'toryline' put after 's' is 'STORYLINE'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for storyline that I've seen before include "Novelly structure" , "Plot of a novel, play or film" .)