Ultimately terrific article probing a strange mystery (7)
I believe the answer is:
arcanum
'mystery' is the definition.
(I've seen this before)
'ultimately terrific article probing a strange' is the wordplay.
'ultimately' says to take the final letters.
'article' becomes 'an' (English indefinite article).
'probing' indicates putting letters inside (probe can mean to poke or prod into).
'strange' becomes 'rum' (similar in meaning).
The last letter of 'terrific' is 'c'.
'c'+'an'='can'
'a'+'rum'='arum'
'can' going within 'arum' is 'ARCANUM'.
(Another definition for arcanum that I've seen is " mystery".)