Lost a letter, I learned (8)
I believe the answer is:
literate
'learned' is the definition.
('literate' can be a synonym of 'learned')
'lost a letter i' is the wordplay.
'lost' indicates anagramming the letters.
'a'+'letter'+'i'='aletteri'
'aletteri' is an anagram of 'LITERATE'.
(Other definitions for literate that I've seen before include "Learned" , "well expressed" , "Erudite" , "version of 'speed of light'?" , "Able to read and write" .)