A fellow's in tiny bits (5)
I believe the answer is:
atoms
'tiny bits' is the definition.
(I've seen this in another clue)
'a fellow's' is the wordplay.
'fellow' becomes 'toms' (I can't justify this - if you can you should believe this answer much more).
'a'+'toms'='ATOMS'
'in' is the link.
Can you help me to learn more?
(Other definitions for atoms that I've seen before include "Jots" , "Small particles" , "Minute particles" , "Tiny particles" , "Dots" .)