Not having any catalogue of things to do results in lethargy (12)
I believe the answer is:
listlessness
'lethargy' is the definition.
(I've seen this before)
'not having any catalogue of things to do' is the wordplay.
I cannot quite see how this works, but
'not' could be 'less' (I've seen this in other clues) and 'less' is found within the answer.
'catalogue' could be 'list' (a catalogue is a list of items) and 'list' is located in the answer.
The remaining letters 'ness' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'results in' acts as a link.
Can you help me to learn more?
(Another definition for listlessness that I've seen is " Lethargy".)