Feast keeps old woman in compound (8)
I believe the answer is:
dopamine
'compound' is the definition.
'feast keeps old woman' is the wordplay.
'feast' becomes 'dine'.
'keeps' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'woman' becomes 'pam' (**).
'o'+'pam'='opam'
'dine' enclosing 'opam' is 'DOPAMINE'.
'in' is the link.
(Other definitions for dopamine that I've seen before include "Neurotransmitter" , "Brain chemical" , "Drug" .)