Stop small object losing its front (4)
I believe the answer is:
stem
'stop' is the definition.
(stem can mean to stop or interrupt)
'small object losing its front' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'object' becomes 'item' (both can mean a thing).
'losing its front' means to remove the first letter.
'item' with its initial letter removed is 'tem'.
's'+'tem'='STEM'
(Other definitions for stem that I've seen before include "Slender vertical part, glass or plant" , "section of pipe" , "part of vegetable?" , "Tube" , "Stop; stalk; ship's timber" .)