End of interminably boring pi? (10,3)
I believe the answer is:
bottomless pit
'end of interminably boring' is the definition.
(boring can mean drilling)
'pi?' is the wordplay.
I cannot quite understand how this works, but
'pi?' could be 'pi' and 'pi' is located in the answer.
This explanation may well be incorrect...
(Other definitions for bottomless pit that I've seen before include "nether regions - with 7a" , "place needing it eternally" , "Source of infinite fuel" .)