Very plain and tough supermarket belt (7)
I believe the answer is:
spartan
'very plain' is the definition.
'spartan' can be an answer for 'plain' (I've seen this before). I am not sure about the 'very' bit.
'tough supermarket belt' is the wordplay.
'tough supermarket' becomes 'spar' (I can't explain this - if you can you should believe this answer much more).
'belt' becomes 'tan' (both can mean to hit).
'spar'+'tan'='SPARTAN'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for spartan that I've seen before include "Not comfortable" , "toughly-disciplined type" , "Lacking comfort or luxury" , "Severe" , "Rather austere" .)