Senselessness of sick needing oxygen getting rolled-up cigarette (7)

'senselessness' is the definition.

'sick needing oxygen getting rolled-up cigarette' is the wordplay.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'needing' is a charade indicator (letters next to each other).
'oxygen getting' becomes 'o' (I can't explain this - if you can you should give a lot more credence to this answer).
'rolled-up' shows that the letters should be reversed in order (in a down clue, letters go up).
'cigarette' becomes 'cig' (short for cigarette).
'cig' in reverse letter order is 'gic'.
'ill'+'o'+'gic'='ILLOGIC'

'of' acts as a link.