Heater used by royal, a woman, and a Conservative mostly (8)
I believe the answer is:
radiator
'heater' is the definition.
(radiator is a kind of heater)
'by royal a woman and a conservative mostly' is the wordplay.
'by royal' becomes 'r' (common abbreviation e.g. RM - Royal Marine. I am not sure about the 'by' bit.).
'woman' becomes 'Di' (short for Diana).
'and' says to put letters next to each other.
'conservative' becomes 'tory' (Tory means a member of the Conservative Party).
'mostly' means to remove the last letter (most but not all of the word).
'tory' with its final letter taken off is 'tor'.
'r'+'a'+'di'+'a'+'tor'='RADIATOR'
'used' is the link.
Can you help me to learn more?
(Other definitions for radiator that I've seen before include "Heater -- or cooler!" , "Domestic heat source" , "Aid or art, strangely, for the heater" , "Domestic heating device" , "Its content is wet!" .)