Pets upset alien group (6)
I believe the answer is:
septet
'group' is the definition.
The answer is an example.
'pets upset alien' is the wordplay.
'upset' indicates anagramming the letters.
'alien' becomes 'et' (extraterrestrial).
'pets' is an anagram of 'sept'.
'sept'+'et'='SEPTET'
(Other definitions for septet that I've seen before include "Group of seven players" , "Group of seven performers or singers" , "piece by Beethoven" , "G7" , "Group of seven voices" .)