So upsetting for the peer, having lost power (9)
I believe the answer is:
therefore
'so' is the definition.
(I've seen this before)
'upsetting for the peer having lost power' is the wordplay.
'upsetting' indicates an anagram.
'having lost' suggests deleting specific letters.
'power' becomes 'p'.
'for'+'the'+'peer'='forthepeer'
'forthepeer' with 'p' removed is 'fortheeer'.
'fortheeer' with letters rearranged gives 'THEREFORE'.
(Other definitions for therefore that I've seen before include "Consequently - for that reason" , "In consequence" , "Ergo" , "Thus" , "It follows" .)