Devious miss might ultimately plot to be mine for evermore? (10,3)
I believe the answer is:
bottomless pit
'mine for evermore?' is the definition.
(mine can mean a pit)
'devious miss might ultimately plot to be' is the wordplay.
'devious' indicates an anagram.
'ultimately' says to take the final letters.
The last letter of 'might' is 't'.
'miss'+'t'+'plot'+'to'+'be'='misstplottobe'
'misstplottobe' is an anagram of 'BOTTOMLESS PIT'.
(Other definitions for bottomless pit that I've seen before include "nether regions - with 7a" , "Source of infinite fuel" , "place needing it eternally" .)