One played with infant: kind of mother! (4,5)
I believe the answer is:
baby grand
'kind of mother' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'one played with infant' is the wordplay.
'one played' becomes 'grand' (I can't justify this - if you can you should give a lot more credence to this answer).
'with' says to put letters next to each other.
'infant' becomes 'baby' (synonyms).
'grand' after 'baby' is 'BABY GRAND'.
Can you help me to learn more?
(Other definitions for baby grand that I've seen before include "Small, non-upright piano" , "Instrument" , "Type of piano" , "Smaller piano" , "Infant piano?" .)