Unit groups must go with purchases by the sound of it (5)
I believe the answer is:
cells
'must go with purchases by the sound of it' is the definition.
I can't tell whether this definition defines the answer.
'unit groups' is the wordplay.
'unit' becomes 'C' (C is an example).
'groups' becomes 'ells' (I can't explain this - if you can you should believe this answer much more).
'c'+'ells'='CELLS'
Can you help me to learn more?
(Other definitions for cells that I've seen before include "Rooms for prisoners" , "Small rooms" , "Accommodation for prisoners" , "terrorist groups" , "Dungeons" .)