Rumpy-pumpy seen in main City gathering (8)
I believe the answer is:
intimacy
'rumpy-pumpy' is the definition.
(jovial term for sexual intercourse)
'main city gathering' is the wordplay.
'gathering' is an anagram indicator (letters gathered in a new way).
'main'+'city'='maincity'
'maincity' with letters rearranged gives 'INTIMACY'.
'seen in' is the link.
(Other definitions for intimacy that I've seen before include "Close friendship" , "close relationship" , "Emotional closeness" , "Close familiarity" , "Sexual intercourse" .)