Son entertained by silly 'alien' pun -- 'one that sticks out on Earth' (9)
I believe the answer is:
peninsula
'one that sticks out on earth' is the definition.
(piece of landing sticking out in the sea)
'son entertained by silly alien pun' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'entertained by' is an insertion indicator (I've seen this in other clues).
'silly' indicates an anagram.
'alien'+'pun'='alienpun'
'alienpun' anagrammed gives 'peninula'.
's' inserted within 'peninula' is 'PENINSULA'.
(Other definitions for peninsula that I've seen before include "Iberia?" , "Piece of land projecting into water" , "Jutting-out strip of land" , "historic war zone" , "Scandinavia, perhaps" .)