"Measure" has one R in it (4)
I believe the answer is:
acre
'measure' is the definition.
(acre is a kind of measure)
'one r in it' is the wordplay.
'one' becomes 'ace' ('ace' can be a synonym of 'one').
'in it' indicates putting letters inside.
'ace' placed around 'r' is 'ACRE'.
'has' acts as a link.
(Other definitions for acre that I've seen before include "holy city" , "Middle East port" , "A state of W Brazil" , "Levant sea port" , "Unit of land area" .)