Fool put up money with horse bolting (6)
I believe the answer is:
nitwit
'fool' is the definition.
(both can mean a stupid person)
'up money with horse bolting' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'money' becomes 'tin' (old-fashioned slang term for money).
'horse' becomes 'H' (abbreviation as in horse power).
'bolting' suggests deleting specific letters (bolt can mean to run away).
'tin' in reverse letter order is 'nit'.
'with' with 'h' removed is 'wit'.
'nit'+'wit'='NITWIT'
'put' acts as a link.
(Other definitions for nitwit that I've seen before include "Blockhead" , "Fool, idiot" , "Silly fool" , "Silly person (colloq.)" , "Louse egg with humour - silly fellow" .)