Sappers left in charge of historical object (5)
I believe the answer is:
relic
'historical object' is the definition.
(I've seen this before)
'sappers left in charge' is the wordplay.
'sappers' becomes 're' (Royal Engineers).
'left' becomes 'l' (common abbreviation).
'in charge' becomes 'ic'.
're'+'l'+'ic'='RELIC'
'of' is the link.
(Other definitions for relic that I've seen before include "Ancient and venerated object" , "Ossified survival" , "Part of a holy person's body or possession" , "Saint's body part?" , "Personal saintly memorial" .)