Brain, for one, that can pull out all the stops (5)
I believe the answer is:
organ
'brain for one that can pull out all the stops' is the definition.
The answer and definition can be both body parts as well as being singular nouns.
Maybe there's an association between them I don't understand?
'brain for one' is the wordplay.
'brain' becomes 'org' (I can't justify this - if you can you should give a lot more credence to this answer).
'for' means one lot of letters go next to another.
'one' becomes 'an' (an apple is one apple).
'org'+'an'='ORGAN'
Can you help me to learn more?
(Other definitions for organ that I've seen before include "Stopped instrument" , "Wind instrument found in the body." , "Functional unit of the body and musical instrument" , "Brain, for example - instrument" , "with a lofty tone?" .)