Silly old peer to show regret (7)
I believe the answer is:
deplore
'regret' is the definition.
(synonyms)
'silly old peer' is the wordplay.
'silly' is an anagram indicator.
'old'+'peer'='oldpeer'
'oldpeer' with letters rearranged gives 'DEPLORE'.
'to show' acts as a link.
(Other definitions for deplore that I've seen before include "Express disgust at" , "Sorrow over" , "Bemoan" , "Regret, censure" , "Regret deeply and strongly disapprove" .)