Some unusual matter in strangely inapt newspaper feature (12)
I believe the answer is:
antiparticle
'some unusual matter' is the definition.
(physics)
'strangely inapt newspaper feature' is the wordplay.
'strangely' is an anagram indicator.
'newspaper feature' becomes 'article' (I've seen this in another clue).
'inapt' with letters rearranged gives 'antip'.
'antip'+'article'='ANTIPARTICLE'
'in' acts as a link.
(Other definitions for antiparticle that I've seen before include "the slightest matter" , "Eg, a positron" , "Positron in relation to an electron, say" , "counterpart to some familiar matter" .)