Bookie covers one fool (3)
I believe the answer is:
sap
'fool' is the definition.
(I know that fool can be written as sap)
'bookie covers one' is the wordplay.
'bookie' becomes 'sp' (I can't explain this - if you can you should believe this answer much more).
'covers' is an insertion indicator.
'one' becomes 'a' (a thing is one thing).
'sp' going around 'a' is 'SAP'.
Can you help me to learn more?
(Other definitions for sap that I've seen before include "Fluid in plant stems" , "Vigour, energy" , "sucker" , "Cosh; plant's juice" , "Exhaust" .)