Starts to lead imposingly, approaching straight (6)
I believe the answer is:
linear
'straight' is the definition.
(I've seen this before)
'starts to lead imposingly approaching' is the wordplay.
'starts to' suggests taking the first letters.
'approaching' becomes 'near' (I've seen this before).
The first letters of 'lead imposingly' is 'li'.
'li'+'near'='LINEAR'
(Other definitions for linear that I've seen before include "Of a line or length" , "Arranged in straight fashion" , "Consisting of lines" , "Involving one dimension only" , "In one dimension, I learn (anag.)" .)