So a chap has a key, as for the piano (6)
I believe the answer is:
sonata
'key as for the piano' is the definition.
I can't tell whether this definition defines the answer.
'so a chap has a' is the wordplay.
'a chap' becomes 'nat' ().
'has' says to put letters next to each other.
'so'+'nat'+'a'='SONATA'
Can you help me to learn more?
(Other definitions for sonata that I've seen before include "Solo instrumental composition" , "Sat on a kind of composition" , "A musical form" , "Moonlight, perhaps" , "piece for instrumental soloist" .)