Old setter's good with child (9)
I believe the answer is:
'with child' is the definition.
(expecting a baby)
'old setter's good' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'setter' becomes 'pectin' (setting agent for jams etc.).
'good' becomes 'g' (abbreviation).
(Other definitions for expecting that I've seen before include "pregnant" , "Regarding as likely to happen" , "Anticipating, looking forward to" .)