This author’s taking in singular theory (3)
I believe the answer is:
ism
'theory' is the definition.
(I have seen 'Distinctive theory' mean 'ism' so perhaps 'theory' could also mean 'ism')
'this author's taking in singular' is the wordplay.
'this author's' becomes 'im' (the person writing the crossword clues).
'taking in' is an insertion indicator.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'im' placed around 's' is 'ISM'.
(Other definitions for ism that I've seen before include "Informally a doctrine, or suffix for one" , "Doctrine, philosophy" , "Suffix denoting doctrine or practice" , "Any ideological doctrine" , "Ideological movement" .)