Detailed plans one can look through (5)
I believe the answer is:
specs
This clue is a double definition.
'detailed plans' is the first definition.
'one can look through' is the second definition.
(one may look using spectacles)
(Other definitions for specs that I've seen before include "Glasses (informal)" , "what builders are doing?" , "Details for designer" , "you can see right through them" , "Glasses (abbr)" .)