Left potential alien transmitted by direct descent (6)
I believe the answer is:
lineal
'transmitted by direct descent' is the definition.
(I've seen this before)
'left potential alien' is the wordplay.
'left' becomes 'l' (common abbreviation**).
'potential' is an anagram indicator.
'alien' anagrammed gives 'ineal'.
'l'+'ineal'='LINEAL'
(Other definitions for lineal that I've seen before include "Having only one dimension" , "In a line as, say, a descendant" , "In line of descendants" , "Transmitted by direct descent" , "One-dimensional" .)