Small tin dog nearly holds relations up (8)
I believe the answer is:
cannikin
'small' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'tin dog nearly holds relations up' is the wordplay.
'tin' becomes 'can' (synonyms).
'dog nearly' becomes 'in' (I can't explain this - if you can you should give a lot more credence to this answer).
'holds' means one lot of letters goes inside another.
'relations' becomes 'kin' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'kin' backwards is 'nik'.
'can'+'in'='canin'
'canin' enclosing 'nik' is 'CANNIKIN'.
Can you help me to learn more?
(Another definition for cannikin that I've seen is " drink from it?".)