Criminal in store looting (7)
I believe the answer is:
pillage
'looting' is the definition.
(thesaurus)
'criminal in store' is the wordplay.
'criminal' becomes 'lag' (slang term for a prisoner).
'in' means one lot of letters goes inside another.
'store' becomes 'pile' (I can't justify this - if you can you should believe this answer much more).
'lag' going into 'pile' is 'PILLAGE'.
Can you help me to learn more?
(Other definitions for pillage that I've seen before include "Violent stealing" , "Plunder, sack" , "Loot" , "Rob with violence" , "Sacking" .)