Ancient mound: yours truly covering over the top (5)
I believe the answer is:
motte
'ancient mound' is the definition.
(historical type of mound)
'yours truly covering over the top' is the wordplay.
'yours truly' becomes 'me' (the person that set this crossword).
'covering' is an insertion indicator.
'over the top' becomes 'OTT' (abbreviation).
'me' placed around 'ott' is 'MOTTE'.
(Other definitions for motte that I've seen before include "Tumulus" , "Rise in defence" , "Castle mound" .)