The endless impulse that accompanies the unknown — it's magic (7)
I believe the answer is:
theurgy
'magic' is the definition.
(theurgy is a kind of magic)
'the endless impulse that accompanies the unknown' is the wordplay.
'endless' means to remove the last letter.
'impulse' becomes 'urge' ('urge' can be a synonym of 'impulse').
'that accompanies' says to put letters next to each other.
'the unknown' becomes 'y' (algebra).
'urge' with its final letter removed is 'urg'.
'the'+'urg'+'y'='THEURGY'
'it's' acts as a link.
(Another definition for theurgy that I've seen is " magic".)