Slowly but surely (4,2,4)
(Other definitions for inch by inch that I've seen before include "Very gradually" , "By small degrees" .)
'slowly but surely' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for steadily that I've seen before include "with regularity" , "Surely and regularly" , "In regular and balanced way" , "Regularly, without fluctuation" , "Regularly, constantly" .)