A large problem grasping second part of mathematics (7)
I believe the answer is:
algebra
'part of mathematics' is the definition.
(I've seen this before)
'a large problem grasping second' is the wordplay.
'problem' indicates an anagram.
'grasping' means one lot of letters goes inside another.
'second' becomes 'b' (B can mean second eg B-side).
'large' with letters rearranged gives 'lgera'.
'lgera' enclosing 'b' is 'lgebra'.
'a'+'lgebra'='ALGEBRA'
(Other definitions for algebra that I've seen before include "Branch of generalised arithmetic" , "Calculation by symbols" , "Form of maths" , "Maths with letters representing numbers" , "Symbol-based branch of mathematics" .)