Human organ left in cup of tea? (9)
I believe the answer is:
earthling
'human' is the definition.
(earthling is a kind of human)
'organ left in cup of tea?' is the wordplay.
'organ' becomes 'ear' (the ear is an organ).
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'cup of tea?' becomes 'thing' (as in 'that's not quite my cup of tea').
'l' inserted within 'thing' is 'thling'.
'ear'+'thling'='EARTHLING'
(Other definitions for earthling that I've seen before include "Human being (sci-fi)" , "man so to be called from space?" , "sci-fi character" , "Man of the world" .)