A spell - but there's nothing in it (4)
I believe the answer is:
bout
'a spell' is the definition.
(I've seen this before)
'but there's nothing in it' is the wordplay.
'there's nothing' becomes 'o' (I've seen this in other clues).
'in it' indicates putting letters inside.
'but' placed around 'o' is 'BOUT'.
(Other definitions for bout that I've seen before include "Attack (of illness)" , "period in ring" , "Contest; illness episode" , "Contest between boxers" , "(Drinking) session" .)