Burial mound yielding up British weapon (5)
I believe the answer is:
arrow
'weapon' is the definition.
(I know that arrow is a type of missile)
'burial mound yielding up british' is the wordplay.
'burial mound' becomes 'barrow' (ancient burial mound).
'yielding up' is a deletion indicator.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'barrow' with 'b' taken away is 'ARROW'.
(Other definitions for arrow that I've seen before include "Archery projectile" , "Keepsake" , "Indicator; missile" , "Narrow missile" , "Dart; pointer" .)