Phosphorus, possibly an atom? (8)
I believe the answer is:
particle
'atom?' is the definition.
(both can mean a small amount of something)
'phosphorus possibly an' is the wordplay.
'phosphorus' becomes 'P' (P is the chemical symbol for phosphorus).
'possibly an' becomes 'article' ('an' is an indefinite article in English).
'p'+'article'='PARTICLE'
(Other definitions for particle that I've seen before include "Grain, tiny piece" , "tiny portion" , "Prelatic (anag.)" , "Eg, a proton" , "Speck of matter" .)