He's brought up just like mum (3)
I believe the answer is:
dad
'mum' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'he's brought up just like' is the wordplay.
'he' becomes 'Ad'.
'brought up' is a reversal indicator (in a down clue, letters go up).
'just like' becomes 'd' (I can't explain this - if you can you should give a lot more credence to this answer).
'ad' in reverse letter order is 'da'.
'da'+'d'='DAD'
Can you help me to learn more?
(Other definitions for dad that I've seen before include "Relation" , "Father, in short" , "Papa" , "Pa" , "Pop, pater" .)