Old blade sat motionless initially having pierced heart (8)
I believe the answer is:
claymore
'old blade' is the definition.
(historical type of sword)
'sat motionless initially having pierced heart' is the wordplay.
'sat' becomes 'lay' (both can mean positioned somewhere).
'initially' suggests taking the first letters.
'having pierced' is an insertion indicator (inserted letters pierce their way in).
'heart' becomes 'core' (synonyms).
The initial letter of 'motionless' is 'm'.
'lay'+'m'='laym'
'laym' placed into 'core' is 'CLAYMORE'.
(Other definitions for claymore that I've seen before include "Scots broadsword" , "Two-edged broadsword" , "mine, sort of" , "Scottish sword wielded by my oracle" , "Highland sword" .)