Foolish to hold son mad (6)
I believe the answer is:
insane
'mad' is the definition.
(I know that mad can be written as insane)
'foolish to hold son' is the wordplay.
'foolish' becomes 'inane' (inane can mean silly or foolish).
'to hold' indicates putting letters inside.
'son' becomes 's' (genealogical abbreviation for son).
'inane' placed around 's' is 'INSANE'.
(Other definitions for insane that I've seen before include "off trolley" , "Wild < cuckoo < cracked" , "Mad, irrational" , "Mentally ill" , "Irresponsible" .)