Former pet can't mix with child (9)
I believe the answer is:
expectant
'with child' is the definition.
(expecting a child)
'former pet can't mix' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'mix' indicates anagramming the letters.
'pet'+'cant'='petcant'
'petcant' is an anagram of 'pectant'.
'ex'+'pectant'='EXPECTANT'
(Other definitions for expectant that I've seen before include "Anticipating" , "Agog" , "Pregnant" , "Hopeful" , "Eager to see what happens" .)