In shopping centres, both change pest repellents (9)
I believe the answer is:
mothballs
'pest repellents' is the definition.
Although both the answer and definition are plural nouns, I don't understand how they can define each other.
'in shopping centres both change' is the wordplay.
'in' means one lot of letters goes inside another.
'shopping centres' becomes 'malls' (I can't explain this - if you can you should give a lot more credence to this answer).
'change' indicates anagramming the letters.
'both' anagrammed gives 'othb'.
'malls' going around 'othb' is 'MOTHBALLS'.
Can you help me to learn more?
(Other definitions for mothballs that I've seen before include "Indefinitely postpones" , "Deterrent" , "Postpones work" , "Camphor spheres" , "stores?" .)