In mathematical solution as well (4)
I believe the answer is:
also
'as well' is the definition.
(synonyms)
'in mathematical solution' is the wordplay.
'in' says the answer is hidden in the clue.
'ALSO' is hidden in the letters of 'mathematical solution'.
(Other definitions for also that I've seen before include "revolutionary a part for Yugoslavia" , "into the bargain" , "on top" , "What's more" , "Plus" .)