Confused agent books for Circle Line? (7)
I believe the answer is:
tangent
'circle line?' is the definition.
(I've seen this in another clue)
'confused agent books' is the wordplay.
'confused' indicates anagramming the letters.
'books' becomes 'NT' (New Testament).
'agent' with letters rearranged gives 'tange'.
'tange'+'nt'='TANGENT'
'for' is the link.
(Other definitions for tangent that I've seen before include "Line touching circumference" , "mostly irrelevant item" , "Straight line just touching a curve" , "Straight line touching a circle" , "Completely different line of thought - trigonometrical function" .)