Pinches a sort of square set askew (6)
I believe the answer is:
tweaks
'pinches' is the definition.
(I know this)
'a sort of square set askew' is the wordplay.
'a sort of square' becomes 'T' (a T-Square is a technical drawing instrument).
'set' indicates an anagram (instruction to set the letters).
'askew' with letters rearranged gives 'weaks'.
't'+'weaks'='TWEAKS'
(Other definitions for tweaks that I've seen before include "Improves a little" , "Some fine tuning" , "Pinches and twists, maybe a nose" , "Makes little adjustments" , "Pinches and twists sharply" .)