Measure many a wild tree (5)
I believe the answer is:
'measure' is the definition.
(I know that measure can be written as meter)
'many a wild tree' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'a wild' is an anagram indicator.
'tree' with letters rearranged gives 'eter'.
(Other definitions for meter that I've seen before include "Device for recording, say, usage (5)" , "Device that measures, say, electricity usage" , "Apparatus for measuring" , "Gauge" , "Measuring device as for electricity usage (5)" .)