Measure head (5)
I believe the answer is:
ruler
I believe this is a double definition.
'measure' is the first definition.
(ruler is a kind of measure)
'head' is the second definition.
(I've seen this before)
(Other definitions for ruler that I've seen before include "Eg, king, emperor" , "Measuring strip" , "Stationery item" , "one going straight?" , "One exercising control" .)