Igneous rock cut at the lowest level (5)
I believe the answer is:
basal
'at the lowest level' is the definition.
(I've seen this before)
'igneous rock cut' is the wordplay.
'igneous rock' becomes 'basalt' (basalt is a kind of igneous rock).
'cut' means to remove the last letter.
'basalt' with its final letter taken away is 'BASAL'.
(Other definitions for basal that I've seen before include "At the lowest level" , "Fundamental" , "forming a foundation" , "bottom layer perhaps" .)