Idle Lee's cooked fish outside (8)
I believe the answer is:
baseless
'idle' is the definition.
(I know that idle can be written as baseless)
'lee's cooked fish outside' is the wordplay.
'cooked' indicates an anagram (letters cooked into a new form).
'fish' becomes 'bass' (bass is a kind of fish).
'outside' is an insertion indicator.
'lees' is an anagram of 'sele'.
'sele' going inside 'bass' is 'BASELESS'.
(Other definitions for baseless that I've seen before include "Without foundation, unsupported" , "Without foundation, unjustified" , "With no foundation in fact" , "Groundless, unfounded" , "Without grounds" .)