Catch an unwary rat on the piano (4)
I believe the answer is:
trap
'catch' is the definition.
(I know that trap is a more specific form of the action catch)
'an unwary rat on the piano' is the wordplay.
'an unwary rat' becomes 'ra' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'piano' becomes 'p' (musical abbreviation).
'ra' put after 't' is 'tra'.
'tra'+'p'='TRAP'
Can you help me to learn more?
(Other definitions for trap that I've seen before include "Part of a snare" , "Means of transport - sometimes sprung!" , "Deceive" , "Rock; carriage; gin" , "Light horse-drawn carriage" .)