Sporting contest, though there's nothing in it (4)
I believe the answer is:
bout
'sporting contest' is the definition.
'bout' can be an answer for 'contest' (I've seen this in other clues). I am not sure about the 'sporting' bit.
'though there's nothing in it' is the wordplay.
'though' becomes 'but' (**).
'there's nothing' becomes 'o' (I've seen this in other clues).
'in it' means one lot of letters goes inside another.
'but' enclosing 'o' is 'BOUT'.
Can you help me to learn more?
(Other definitions for bout that I've seen before include "Fit" , "Fight; instance of illness" , "Wrestling contest" , "Spell - boxing match" , "Spell of eg drinking" .)