Left in, ahead of time, in a straight line (8)
I believe the answer is:
linearly
'in a straight line' is the definition.
(I've seen this before)
'left in ahead of time' is the wordplay.
'left' becomes 'l' (common abbreviation).
'ahead of time' becomes 'early' (synonyms).
'l'+'in'+'early'='LINEARLY'
(Other definitions for linearly that I've seen before include "In a straight line" , "in sequence" , "in elongated mode" .)