Hit 100 thousand! (4)
I believe the answer is:
tonk
'hit' is the definition.
(tonk can mean to hit hard)
'100 thousand' is the wordplay.
'k00' becomes 'ton' (ton can informally mean a hundred).
'thousand' becomes 'k' (synonyms).
'ton'+'k'='TONK'
(Other definitions for tonk that I've seen before include "from Australia is hit" , "Hit hard" , "Rod" .)