Really small boat has music playing (9)
I believe the answer is:
subatomic
'really small' is the definition.
'subatomic' can be an answer for 'small' (similar in meaning). I'm not sure about the 'really' bit.
'boat has music playing' is the wordplay.
'has' says to put letters next to each other.
'playing' indicates anagramming the letters.
'boat'+'music'='boatmusic'
'boatmusic' with letters rearranged gives 'SUBATOMIC'.
Can you help me to learn more?
(Other definitions for subatomic that I've seen before include "Of smaller than elementary particles" , "Smaller than a basic particle of matter" , "Like 9 across" .)