Measuring stick (5)
'measuring stick' is the definition.
(ruler is a kind of measuring stick)
This is all the clue.
(Other definitions for ruler that I've seen before include "Person in authority" , "Straight edge; monarch" , "Person in charge of country" , "Potentate" , "Stationery item" .)
(Other definitions for gauge that I've seen before include "Adjudge" , "Rail track width" , "Measure(r)" , "Measure - estimate" , "standard size" .)