Prune it, just a bit, and you'll make an impression (3,1,4)
I believe the answer is:
cut a dash
'make an impression' is the definition.
(I've seen this before)
'prune it just a bit and you'll' is the wordplay.
I cannot quite see how this works, but
'prune' could be 'cut' (synonyms) and 'cut' is found within the answer.
'a' is found in the answer.
The remaining letters 'dash' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cut a dash that I've seen before include "look smart" , "edit out hyphen" , "make a big impression" , "How to produce a hyphen" , "Have a striking look" .)