Sit alone twiddling lower extremities (8)
I believe the answer is:
Here is my best explanation:
'lower extremities' is the definition.
'sit alone twiddling' is the wordplay.
'twiddling' is an anagram indicator.
'sitalone' is an anagram of 'TOENAILS'.
(Other definitions for toenails that I've seen before include "types of digital hardware" and "Extremities".)