Mourn eccentric old peer (7)
I believe the answer is:
deplore
'mourn' is the definition.
(thesaurus)
'eccentric old peer' is the wordplay.
'eccentric' indicates anagramming the letters.
'old'+'peer'='oldpeer'
'oldpeer' with letters rearranged gives 'DEPLORE'.
(Other definitions for deplore that I've seen before include "Sorrow over" , "Lament" , "Express disgust at" , "Regret deeply and strongly disapprove" , "Bemoan" .)