SuttaCentral
Hello. I’m asking for help trying to locate the sentence in the title. I’ve not been successful. I heard it was in MN but haven’t found it yet.
Also could this be translated as “wise attention is the one thing that can see rightly”. Not looking for precision, just looking for the ball park at the moment.
Thank you 
I think it’s likely to be from the series of short suttas about mental qualities in the AN1’s that are “one thing” or “a single thing”. Not quite the same as your text but fairly similar:
“Mendicants, I do not see a single thing that prevents doubt from arising, or, when it has arisen, gives it up like rational application of mind.
“Nāhaṁ, bhikkhave, aññaṁ ekadhammampi samanupassāmi yena anuppannā vā vicikicchā nuppajjati uppannā vā vicikicchā pahīyati yathayidaṁ, bhikkhave, yonisomanasikāro.
When you apply the mind rationally, doubt does not arise, or, if it’s already arisen, it’s given up.”
Yoniso, bhikkhave, manasi karoto anuppannā ceva vicikicchā nuppajjati uppannā ca vicikicchā pahīyatī”ti.
You might try searching for Bhante Sujato’s translation of yoniso manasikhara , “rational application of mind”.
The closest I can think of is:
Dve kho, āvuso, paccayā sammādiṭṭhiyā uppādāya parato ca ghoso, yoniso ca manasikāro.
There are two conditions for the arising of right view: the words of another and rational application of mind.
Searching around the closest I can find is:
Katamo eko dhammo visesabhāgiyo? Yoniso manasikāro.
What one thing leads to distinction? Rational application of mind.
Thank you Bhante. Grateful for your help. I’ve been reading Bhante Sujato’s posts on yoniso manasikāra today 
Thank you for the suggestions Bhante.