One tucked into relation's cereal (5)
I believe the answer is:
grain
'cereal' is the definition.
('grain' can be a synonym of 'cereal')
'one tucked into relation's' is the wordplay.
'one' becomes 'i' (Roman numeral).
'tucked into' indicates putting letters inside.
'relation' becomes 'gran' (I've seen this before**).
'i' inserted into 'gran' is 'GRAIN'.
(Other definitions for grain that I've seen before include "Direction of fibres found in wood" , "basis for whisky" , "bit of corn" , "Cereal seed" , "little salty bit" .)