Pets reforming alien group (6)
I believe the answer is:
septet
'group' is the definition.
The answer is an example.
'pets reforming alien' is the wordplay.
'reforming' indicates anagramming the letters.
'alien' becomes 'et' (extra terrestrial).
'pets' anagrammed gives 'sept'.
'sept'+'et'='SEPTET'
(Other definitions for septet that I've seen before include "Heptad" , "Snow White's companions?" , "Musical group" , "Music piece for seven" , "G7" .)