Mark dead on target, initially (3)
I believe the answer is:
dot
'mark' is the definition.
(dotting is a kind of marking)
'dead on target initially' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'dead on target' is 'DOT'.
(Other definitions for dot that I've seen before include "Small round spot" , "Spot, speck" , "Point; speck" , "'Spot, tiny mark (3)'" , "Full stop, period or tittle" .)