To play only about half a duet is rather poor (4,2)
I believe the answer is:
hard up
'rather poor' is the definition.
'hard up' can be an answer for 'poor' (similar in meaning). I'm not sure about the 'rather' bit.
'to play only about half a duet' is the wordplay.
'to play only' becomes 'harp' (I can't explain this - if you can you should believe this answer much more).
'about' is an insertion indicator.
'half a' suggests halving the letters.
'duet' with half the letters taken is 'du'.
'harp' enclosing 'du' is 'HARD-UP'.
'is' is the link.
Can you help me to learn more?
(Other definitions for hard up that I've seen before include "Panto Baron; needy" , "Out of funds - it's tough" , "Having little money, almost skint" , "Struggling with budget" , "Badly-off" .)