Inventor lacking edge retrospectively (6)
I believe the answer is:
edison
'inventor' is the definition.
(I know that Edison is an inventor)
'lacking edge retrospectively' is the wordplay.
'lacking' becomes 'no' (I've seen this in other clues).
'edge' becomes 'side' (edge can mean the side of something).
'retrospectively' is a reversal indicator.
'no'+'side'='noside'
'noside' written backwards gives 'EDISON'.
(Other definitions for edison that I've seen before include "Thomas ......, famous American inventor" , "Early American inventor" , "US scientist/inventor" , "Science master" , "US inventor of over 1,000 devices" .)