Measure. It's a yard (4)
I believe the answer is:
area
'yard' is the definition.
(thesaurus)
'measure it's a' is the wordplay.
'measure' becomes 'are' (are is a kind of measure**).
'are'+'a'='AREA'
(Other definitions for area that I've seen before include "'Tract, region (4)'" , "Territory" , "Surface measurement" , "Patch" , "environs" .)