Lad, having died within, is corpse (4)
I believe the answer is:
body
'corpse' is the definition.
(I've seen this before)
'lad having died within' is the wordplay.
'lad' becomes 'boy' (I've seen this before).
'having' is an insertion indicator.
'died within' becomes 'd' (abbreviation next to the year of someone's death. I am not sure about the 'within' bit.).
'boy' placed around 'd' is 'BODY'.
'is' is the link.
Can you help me to learn more?
(Other definitions for body that I've seen before include "Non-verbal communication?" , "Stiff" , "Individual" , "Hare or Burke, for example" , "Corpse; main part" .)