Last in House by virtue of being Liberal peer (5)
I believe the answer is:
equal
'peer' is the definition.
(synonyms)
'last in house by virtue of being liberal' is the wordplay.
'last in' says to take the final letters.
'by virtue of being' becomes 'qua' (Latin word meaning 'in the capacity of' or 'as').
'liberal' becomes 'l' (abbreviation).
The last letter of 'house' is 'e'.
'e'+'qua'+'l'='EQUAL'
(Other definitions for equal that I've seen before include "Identical, add up to" , "The same in quantity, status etc" , "The same in size, amount or value, etc." , "Level" , "Peer; evenly balanced" .)