Frenchman with new tree to gauge (5)
I believe the answer is:
meter
'to gauge' is the definition.
(I've seen this before)
'frenchman with new tree' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur).
'with' says to put letters next to each other.
'new' is an anagram indicator.
'tree' anagrammed gives 'eter'.
'm'+'eter'='METER'
(Other definitions for meter that I've seen before include "Device for measuring and recording" , "Gauge" , "Instrument for measuring quantity" , "Device to measure quantity used of fluid, electricity etc" , "Something measuring" .)