Thrash bloke producing geometric line (7)
I believe the answer is:
tangent
'geometric line' is the definition.
(I've seen this before)
'thrash bloke' is the wordplay.
'thrash' becomes 'tan' (I've seen this before).
'bloke' becomes 'gent' (I've seen this in other clues).
'tan'+'gent'='TANGENT'
'producing' acts as a link.
(Other definitions for tangent that I've seen before include "What people often go off at!" , "Line touching curve at one point" , "Straight line just touching a curve" , "Geometric line" , "seen to be touching" .)