Little bit of the first half of the alphabet (4)
I believe the answer is:
atom
I believe this is a triple definition!
'little' is the first definition.
(thesaurus)
'bit' is the second definition.
(I've seen this in another clue)
'of' acts as a link.
'the first half of the alphabet' is the third definition.
(I've seen this in another clue)
(Other definitions for atom that I've seen before include "A tiny particle but a big bomb" , "Basic unit of a chemical element" , "Smallest particle in an element able to take part in a chemical reaction" , "Smallest unit of matter" , "Part of molecule" .)