Concert pianist extremely quick to respond (6)
I believe the answer is:
prompt
'quick to respond' is the definition.
I can't tell whether this definition defines the answer.
'concert pianist extremely' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'extremely' suggests removing the centre (only the extremes of the word are used).
'pianist' with its centre removed is 'pt'.
'prom'+'pt'='PROMPT'
Can you help me to learn more?
(Other definitions for prompt that I've seen before include "Immediate - supply actor's lines" , "Cue given to a performer" , "Encourage" , "Help forgetful actor with lines" , "Quick, without delay" .)