Starts to lead imposingly, approaching straight (6)
I believe the answer is:
Here is my best explanation:
'straight' is the definition.
'starts to lead imposingly approaching' is the subsidiary indication.
'starts to' suggests taking the first letters.
'approaching' becomes 'near' (I've seen this before).
The first letters of 'lead imposingly' is 'li'.
(Other definitions for linear that I've seen before include "Of a line or length", "Like some ancient scripts", "following straight path", "Long and narrow, of one dimension", "One-dimensional", "Straight; sequential", "In a straight line", "-- Motor; -- equation", "Nailer (anag.)" and "Of one dimension".)