Alien encounters principal character, and so on (3)
I believe the answer is:
etc
'and so on' is the definition.
(I've seen this before)
'alien encounters principal character' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'encounters' means one lot of letters go next to another.
'principal character' becomes 'c' (I am not sure about the 'principal' bit.).
'et'+'c'='ETC'
Can you help me to learn more?
(Other definitions for etc that I've seen before include "And so on, in brief" , "'And the rest, in short (3)'" , "Etcetera (abbrev.)" , "And so on, briefly" , "And so forth (abbr)" .)