Spear as good in main incursion, initially (7)
I believe the answer is:
assegai
'spear' is the definition.
(I know that assegai is a type of spear)
'as good in main incursion initially' is the wordplay.
'good' becomes 'g' (abbreviation).
'in' means one lot of letters goes inside another.
'main' becomes 'sea' (the main is an old name for the sea).
'initially' indicates taking the first letters.
The first letter of 'incursion' is 'i'.
'g' put inside 'sea' is 'sega'.
'as'+'sega'+'i'='ASSEGAI'
(Other definitions for assegai that I've seen before include "Ages as I threw the spear" , "Spear of the Bantu-speaking people of Africa" , "spear made from wood" , "African weapon" , "Zulu fighting spear" .)