Crooked agent books for Circle Line? (7)
I believe the answer is:
tangent
'circle line?' is the definition.
(I've seen this in another clue)
'crooked agent books' is the wordplay.
'crooked' is an anagram indicator (a crooked or disrupted version of the letters).
'books' becomes 'NT' (abbreviation for New Testament).
'agent' anagrammed gives 'tange'.
'tange'+'nt'='TANGENT'
'for' is the link.
(Other definitions for tangent that I've seen before include "Particular line" , "What people often go off at!" , "Line grazing another" , "Straight line that touches but does not intersect a curve" , "seen to be touching" .)