I'd most hatred, on and off, for old fool (6)
I believe the answer is:
dotard
'old fool' is the definition.
(an old person whose mental faculties have declined)
'i'd most hatred on and off' is the wordplay.
'on and off' means one should take alternating letters (letters selected on and off at regular intervals).
'id'+'most'+'hatred'='idmosthatred'
The alternate letters of 'idmosthatred' are 'DOTARD'.
'for' is the link.
(Other definitions for dotard that I've seen before include "Old fogey" , "Person old and feeble" , "Weak old person" , "Person who is weak-minded, especially through senility" , "Person weakened in mind by age" .)