Gauge medium tree to be deformed (5)
I believe the answer is:
meter
'gauge' is the definition.
(I know that gauge can be written as meter)
'medium tree to be deformed' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'to be deformed' indicates an anagram.
'tree' anagrammed gives 'eter'.
'm'+'eter'='METER'
(Other definitions for meter that I've seen before include "Measuring device as for electricity usage (5)" , "Device to measure quantity used of fluid, electricity etc" , "Measuring device, say for electricity usage" , "Device for measuring and recording" , "Something measuring" .)