Mark cryptic clue for tiny thing (8)
I believe the answer is:
molecule
'tiny thing' is the definition.
(a molecule is a group of atoms)
'mark cryptic clue' is the wordplay.
'mark' becomes 'mole' (mark on the skin).
'cryptic' is an anagram indicator (I've seen this in other clues).
'clue' anagrammed gives 'cule'.
'mole'+'cule'='MOLECULE'
'for' acts as a link.
(Other definitions for molecule that I've seen before include "Smallest independently existing thing" , "Smallest particle of a substance" , "part of compound" , "Group of atoms bonded together" , "... One unit of alcohol perhaps" .)