Conservative supporting a nobleman's explanation (7)
I believe the answer is:
account
'explanation' is the definition.
(synonyms)
'conservative supporting a nobleman's' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'nobleman' becomes 'count' (count is a kind of nobleman).
'c' after 'a' is 'ac'.
'ac'+'count'='ACCOUNT'
(Other definitions for account that I've seen before include "Explanation of conduct" , "Narrative; statement" , "Story; bank service" , "Statement of transactions and costs" , "Tell" .)