Give an account of soldiers deceased (6)
I believe the answer is:
Here is my best explanation:
'give an account' is the definition.
(I've seen this before)
'soldiers deceased' is the subsidiary indication.
'soldiers' becomes 're' (Royal Engineers).
'deceased' becomes 'late' (I've seen this in other clues).
'of' is the link.
(Other definitions for relate that I've seen before include "Make a logical or casual connection", "Show sympathy", "Connect in Tralee", "Do well; wave about", "Narrate, tell in detail", "Recite - marriage guidance organisation", "Narrate or associate", "Marriage Guidance - report", "Give account of (story)" and "Empathise".)