Part of mathematical solution in addition (4)
I believe the answer is:
also
'in addition' is the definition.
('also' can be a synonym of 'in addition')
'part of mathematical solution' is the wordplay.
'part of' indicates a hidden word.
'ALSO' is hidden in the letters of 'mathematical solution'.
(Other definitions for also that I've seen before include "To boot" , "Old" , "on top" , "an indication of more to come" , "'Besides, moreover (4)'" .)