Eggs sat on by considerate male penguin (6)
I believe the answer is:
gentoo
'penguin' is the definition.
(I've seen this before)
'eggs sat on by considerate male' is the wordplay.
'eggs' becomes 'oo' (two O's - O looks like an egg).
'sat on by' is a charade indicator (letters next to each other).
'considerate male' becomes 'gent' (gent is a kind of male. I am not sure about the 'considerate' bit.).
'oo' after 'gent' is 'GENTOO'.
Can you help me to learn more?
(Other definitions for gentoo that I've seen before include "bird from the Falklands" , "Variety of penguin" , "penguin on Goose Green!" , "remote island penguin" , "Breed of penguin" .)