An infinitesimally small unit I say I see is physically structural (8)
I believe the answer is:
anatomic
'structural' is the definition.
(I've seen this in another clue)
'an infinitesimally small' is the wordplay.
'an' becomes 'a'.
'infinitesimally' becomes 'n' (I can't justify this - if you can you should believe this answer much more).
'small' becomes 'atomic' (similar in meaning).
'a'+'n'+'atomic'='ANATOMIC'
'unit i say i see is physically' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for anatomic that I've seen before include "Relating to body structure" , "Relating to bodily structure" , "Studying the structure of people" , "associated with a body" .)