Perpendicular cut, say (5)
I believe the answer is:
sheer
'perpendicular' is the definition.
(synonyms)
'cut say' is the wordplay.
'cut' becomes 'shear' (shearing is a kind of cutting).
'say' indicates a 'sounds like' (homophone) clue.
'shear' sounds like 'SHEER'.
(Other definitions for sheer that I've seen before include "Clear" , "Diaphanous, filmy" , "Absolute - precipitous" , "needing to change a course" , "Arrant" .)