Shift key regularly errs (5)
I believe the answer is:
alter
'shift' is the definition.
(I've seen this before)
'key regularly errs' is the wordplay.
'key' becomes 'alt' (the Alt keyboard button).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'errs' are 'er'.
'alt'+'er'='ALTER'
(Other definitions for alter that I've seen before include "Revise" , "Change, modify" , "Change or adjust" , "Alert to change" , "Change, make different" .)