Gauge medium tree differently (5)
I believe the answer is:
meter
'gauge' is the definition.
(I've seen this in another clue)
'medium tree differently' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'differently' indicates an anagram.
'tree' with letters rearranged gives 'eter'.
'm'+'eter'='METER'
(Other definitions for meter that I've seen before include "Device for recording, say, usage (5)" , "Apparatus for measuring" , "Measuring device, say for electricity usage" , "Something measuring" , "Device to measure quantity used of fluid, electricity etc" .)