Maths problem that is back in shows lack of belief (7)
I believe the answer is:
atheism
'lack of belief' is the definition.
'maths problem that is back in' is the wordplay.
'problem' is an anagram indicator.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'back' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'ie' backwards is 'ei'.
'maths' is an anagram of 'athsm'.
'athsm' going around 'ei' is 'ATHEISM'.
'shows' acts as a link.
(Other definitions for atheism that I've seen before include "A lack of faith" , "Owed" , "Sceptic's position" , "Non-belief in God" , "Disbelief in a god" .)