It's sheer nonsense taking time off (6)
I believe the answer is:
rotten
'it's sheer' is the definition.
I can't tell whether this definition defines the answer.
'nonsense taking time off' is the wordplay.
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'taking' is a charade indicator (letters next to each other).
'time off' becomes 'ten' (I've seen this in another clue. I am not sure about the 'off' bit.).
'rot'+'ten'='ROTTEN'
Can you help me to learn more?
(Other definitions for rotten that I've seen before include "Corrupt; very bad" , "Morally despicable" , "Decaying - quite putrid" , "Something is ... in the state of Denmark (Hamlet)" , "Lousy" .)