One of the subjects reviews The Imposter outside the capital (5)
I believe the answer is:
maths
'one of the subjects' is the definition.
'maths' can be an answer for 'subjects' (math is a kind of subject). I'm not sure about the 'one of the' bit.
'reviews the imposter outside the capital' is the wordplay.
'reviews' is a reversal indicator (view the letters another way).
'the imposter' becomes 'sham' ('sham' can be a synonym of 'imposter').
'outside' indicates putting letters inside.
'capital' says to take the initial letters.
The first letter of 'the' is 't'.
'sham' going around 't' is 'shtam'.
'shtam' back-to-front is 'MATHS'.
Can you help me to learn more?
(Other definitions for maths that I've seen before include "Numbers subject" , "Adding and subtraction, etc" , "Study of numbers and sums" , "Algebra, geometry, calculus etc (abbr)" , "Subject involving figures (abbr)" .)