Reason to speak about overturned container (6)
I believe the answer is:
sanity
'reason' is the definition.
(thesaurus)
'to speak about overturned container' is the wordplay.
'to speak' becomes 'say' (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'overturned' says the letters should be written in reverse.
'container' becomes 'tin' (tin is a kind of container**).
'tin' reversed gives 'nit'.
'say' going around 'nit' is 'SANITY'.
(Other definitions for sanity that I've seen before include "Balance of mind" , "Reason and coherence" , "Sound powers of mind" , "Healthy mental state" , "Mental health" .)