Very different topic has fellow on strike (7)
I believe the answer is:
tangent
'very different topic' is the definition.
(as in going off on a tangent)
'fellow on strike' is the wordplay.
'fellow' becomes 'gent' (synonyms).
'on' is a charade indicator (letters next to each other).
'strike' becomes 'tan' (both can mean to hit).
'gent' put after 'tan' is 'TANGENT'.
'has' is the link.
(Other definitions for tangent that I've seen before include "Straight line that touches but does not intersect a curve" , "Straight line just touching a curve" , "Line grazing another" , "Line touching curve at one point" , "Straight line that touches but does not intersect a circle" .)