Mathematical array in jumble involving fiendish art (6)
I believe the answer is:
matrix
'mathematical array' is the definition.
(I've seen this before)
'jumble involving fiendish art' is the wordplay.
'jumble' becomes 'mix'.
'involving' indicates putting letters inside.
'fiendish' is an anagram indicator (an unexpected or fiendish version of the letters).
'art' is an anagram of 'atr'.
'mix' enclosing 'atr' is 'MATRIX'.
'in' is the link.
(Other definitions for matrix that I've seen before include "Frame - template" , "Casting mould" , "Mathematical array of numbers" , "network of circuit elements" , "Arrangement of numbers" .)