Pack it in jars to be sent back (4)
I believe the answer is:
stop
'pack' is the definition.
(thesaurus)
'it in jars to be sent back' is the wordplay.
'it' becomes 't' (abbreviation. e.g. in 'tis).
'in' indicates putting letters inside.
'jars' becomes 'pos' (I can't explain this - if you can you should give a lot more credence to this answer).
'to be sent back' says the letters should be written backwards.
't' going into 'pos' is 'pots'.
'pots' reversed gives 'STOP'.
Can you help me to learn more?
(Other definitions for stop that I've seen before include "Red light signal" , "Come to a halt" , "Halt; finish" , "Intercept" , "Come to an end" .)