Having nothing to do, one gets led astray (4)
I believe the answer is:
idle
'having nothing to do' is the definition.
(I've seen this before)
'one gets led astray' is the wordplay.
'one' becomes 'i' (Roman numeral).
'gets' is an insertion indicator.
'astray' indicates an anagram.
'led' anagrammed gives 'dle'.
'i' placed within 'dle' is 'IDLE'.
(Other definitions for idle that I've seen before include "-- Jack, pantomime character" , "Trivial" , "Frivolous" , "not working in > lounge" , "Not occupied" .)