Relative dunce is soon struggling over calculus primarily (6,6)
I believe the answer is:
second cousin
'relative' is the definition.
(second cousin is a kind of relative)
'dunce is soon struggling over calculus primarily' is the wordplay.
'struggling' is an anagram indicator (the letters struggle or wriggle into a new form).
'over' is an insertion indicator.
'primarily' says to take the initial letters.
The first letter of 'calculus' is 'c'.
'dunce'+'is'+'soon'='dunceissoon'
'dunceissoon' with letters rearranged gives 'secondousin'.
'secondousin' going around 'c' is 'SECOND COUSIN'.
(Other definitions for second cousin that I've seen before include "Some family" , "Fairly distant relation" , "Relative" .)