Devotion to ally in tangle with unknown quantity (7)
I believe the answer is:
loyalty
'devotion' is the definition.
(I've seen this in another clue)
'to ally in tangle with unknown quantity' is the wordplay.
'in tangle with' indicates anagramming the letters.
'unknown quantity' becomes 'y' (algebra).
'to'+'ally'='toally'
'toally' anagrammed with 'y' makes 'LOYALTY'.
(Other definitions for loyalty that I've seen before include "Faithfully allegiance" , "Allegiance, commitment" , "Strong allegiance" , "Dedication" , "Firm allegiance" .)