Tell about nobleman (7)
I believe the answer is:
recount
'tell' is the definition.
(synonyms)
'about nobleman' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'nobleman' becomes 'count' (count is a type of aristocrat).
're'+'count'='RECOUNT'
(Other definitions for recount that I've seen before include "reason for lateness of election result?" , "Check numbers" , "electoral process" , "result of close-run ballot" , "post-election procedure?" .)