Cooking up eats with a grannie in her eighth decade? (14)
I believe the answer is:
septuagenarian
'in her eighth decade?' is the definition.
'cooking up eats with a grannie' is the wordplay.
'cooking' is an anagram indicator.
'with' means one lot of letters go next to another.
'up'+'eats'+'a'+'grannie'='upeatsagrannie'
'upeatsagrannie' with letters rearranged gives 'SEPTUAGENARIAN'.
(Other definitions for septuagenarian that I've seen before include "OAP" , "pensioner" , "Elderly person" , "One aged 70 to 79" .)