Sinister rising member of upper house cutting Young Conservatives (6)
I believe the answer is:
creepy
'sinister' is the definition.
(I've seen this before)
'rising member of upper house cutting young conservatives' is the wordplay.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'member of upper house' becomes 'peer' (member of the House of Lords).
'cutting' means one lot of letters goes inside another (some letters cut their way into a word).
'young' becomes 'y'.
'conservatives' becomes 'C' (abbreviation in UK politics).
'y'+'c'='yc'
'peer' put into 'yc' is 'ypeerc'.
'ypeerc' written backwards gives 'CREEPY'.
(Other definitions for creepy that I've seen before include "repulsive" , "Annoying and unpleasant" , "Eerie - chilling" , "Threatening" , "Causing a feeling of fear or unease" .)