Frenchman used tree as a measure (5)
I believe the answer is:
metre
'a measure' is the definition.
(I know that measure can be written as metre)
'frenchman used tree' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'used' indicates anagramming the letters.
'tree' anagrammed gives 'etre'.
'm'+'etre'='METRE'
'as' is the link.
(Other definitions for metre that I've seen before include "Rhythmic arrangement of syllables in verse" , "Yard is smaller than this" , "Approximately 39 inches" , "100cm" , "determined by scanning" .)