Frenchman has new tree to gauge (5)
I believe the answer is:
meter
'to gauge' is the definition.
(I've seen this in another clue)
'frenchman has new tree' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur).
'has' means one lot of letters go next to another.
'new' indicates anagramming the letters.
'tree' is an anagram of 'eter'.
'm'+'eter'='METER'
(Other definitions for meter that I've seen before include "Poe sometimes used this" , "Device for recording, say, usage (5)" , "Device to measure quantity used of fluid, electricity etc" , "Apparatus for measuring" , "Gauge" .)