Cry because it's only a baby piano (4)
I believe the answer is:
weep
'cry' is the definition.
(synonyms)
'baby piano' is the wordplay.
'baby' becomes 'wee' (I've seen this in another clue).
'piano' becomes 'p' (musical abbreviation).
'wee'+'p'='WEEP'
'because it's only a' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for weep that I've seen before include "Seep slowly out" , "pipe" , "Display sorrow" , "Keen" , "leak" .)