Sally's about to finish up fast! (5)
I believe the answer is:
rapid
'fast' is the definition.
(similar in meaning)
'sally's about to finish up' is the wordplay.
'sally' becomes 'raid' (both can mean a military attack).
'about' indicates putting letters inside.
'to finish' means to take half the letters.
'up' halved is 'p'.
'raid' going around 'p' is 'RAPID'.
(Other definitions for rapid that I've seen before include "Fast, speedy" , "Flying" , "Happening quickly" , "Fleet, swift" , "Quick or swift" .)