Pets set off in groups (7)
I believe the answer is:
septets
'in groups' is the definition.
'septets' can be an answer for 'groups' (I have seen 'Groups of seven ' mean 'septets' so perhaps 'groups' could also mean 'septets'). I'm unsure of the 'in' bit.
'pets set off' is the wordplay.
'off' indicates an anagram.
'pets'+'set'='petsset'
'petsset' anagrammed gives 'SEPTETS'.
Can you help me to learn more?
(Other definitions for septets that I've seen before include "Musicians" , "Groups of seven" .)