Be left behind again by an inventor (4)
I believe the answer is:
bell
'an inventor' is the definition.
(Alexander Graham Bell)
'be left behind again' is the wordplay.
'be left' becomes 'L' (common abbreviation).
'behind' is a charade indicator (letters next to each other).
'again' becomes 'bel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'l' put after 'bel' is 'BELL'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for bell that I've seen before include "Scottish-born inventor of the telephone, d. 1922" , "kind of curve" , "End of oboe; inventor of telephone" , "pen name" , "Alexander ..., inventor of the telephone" .)