An atom is too small for one who studies body structure (9)
I believe the answer is:
anatomist
'one who studies body structure' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'an atom is too small' is the wordplay.
'is too small' becomes 'ist' (I can't justify this - if you can you should give a lot more credence to this answer).
'an'+'atom'+'ist'='ANATOMIST'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for anatomist that I've seen before include "Biological expert" , "One studying bodily structure" , "Body expert" , "Expert in the structure of human and animal bodies" , "student of structure" .)