Stop half of them on the way (4)
I believe the answer is:
stem
'stop' is the definition.
(stemming is a kind of stopping)
'half of them on the way' is the wordplay.
'half of' indicates taking half.
'on' says to put letters next to each other.
'the way' becomes 'st'.
'them' with half the letters taken is 'em'.
'em' after 'st' is 'STEM'.
(Other definitions for stem that I've seen before include "Basic skiing turn" , "Staunch - support" , "Peduncle" , "Vertical part of flower or drinking glass" , "Stop; stalk; ship's timber" .)