Printers measure newspapers with a ruler (7)
I believe the answer is:
empress
'ruler' is the definition.
(female emperor)
'printers measure newspapers' is the wordplay.
'printers measure' becomes 'em' (I am not sure about this - if you are sure you should believe this answer much more).
'newspapers' becomes 'press' (the press is the newspaper industry).
'em'+'press'='EMPRESS'
'with a' acts as a link.
Can you help me to learn more?
(Other definitions for empress that I've seen before include "Delilah?" , "She rules a vast realm" , "leading lady" , "In Victoria, say" , "Maria Theresa, perhaps" .)