A neat junction at the margin (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
trim
'a neat' is the definition.
(I've seen this before)
'junction at the margin' is the wordplay.
'junction' becomes 't'.
'at' says to put letters next to each other.
'the margin' becomes 'rim' (both can mean something's edge).
't'+'rim'='TRIM'
(Other definitions for trim that I've seen before include "Neat - prune" , "Edging material - in good order" , "(Make) neat and tidy" , "Prune - spruce" , "Svelte" .)