It's infernally bad when he gets left behind again (4)
I believe the answer is:
hell
'it's infernally bad when he' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'left behind again' is the wordplay.
'left' becomes 'l' (common abbreviation).
'behind' says to put letters next to each other.
'again' becomes 'hel' (I can't justify this - if you can you should give a lot more credence to this answer).
'l' put after 'hel' is 'HELL'.
'gets' is the link.
Can you help me to learn more?
(Other definitions for hell that I've seen before include "Gehenna" , "uproar" , "Hades, the netherworld" , "'The road to . . . . is paved with good intentions'" , "Infernal place" .)