Subject finally caught withdrawing fake cases (5)
I believe the answer is:
maths
'subject' is the definition.
(maths is a kind of subject)
'finally caught withdrawing fake cases' is the wordplay.
'finally caught' becomes 't' (final letter of 'caught').
'withdrawing' shows that the letters should be reversed in order.
'fake' becomes 'sham' (sham can mean fake or pretend).
'cases' is an insertion indicator.
'sham' written backwards gives 'mahs'.
't' put within 'mahs' is 'MATHS'.
(Other definitions for maths that I've seen before include "subject, in short" , "Study of numbers (abbr.)" , "It concerns numbers, in short" , "The science of number, quantity and space" , "Science dealing with quantity and shapes, in short" .)