Stupid and senseless to incarcerate son (6)
I believe the answer is:
insane
'stupid' is the definition.
(I've seen this before)
'senseless to incarcerate son' is the wordplay.
'senseless' becomes 'inane' (I've seen this before).
'to incarcerate' means one lot of letters goes inside another.
'son' becomes 's'.
'inane' going around 's' is 'INSANE'.
'and' is the link.
(Other definitions for insane that I've seen before include "Mentally unhinged" , "reckless" , "Demented" , "Eg, gun or knife" , "Mad, irrational" .)