Sally admitting tip of pencil's sharp (5)
I believe the answer is:
rapid
'sharp' is the definition.
(thesaurus)
'sally admitting tip of pencil's' is the wordplay.
'sally' becomes 'raid' (both can mean a military attack).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'tip of' says to take the initial letters.
The initial letter of 'pencils' is 'p'.
'raid' enclosing 'p' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Fast, speedy" , "Happening quickly" , "Moving at speed" , "Fleet, swift" , "haste" .)