Regularly crude sign (3)
I believe the answer is:
cue
'sign' is the definition.
(thesaurus)
'regularly crude' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'crude' are 'CUE'.
(Other definitions for cue that I've seen before include "Used in snooker and on stage" , "line in speech" , "Reminder to do something" , "Prompt; reminder" , "signal to come on" .)