Step, missing a beat (4)
I believe the answer is:
stir
'beat' is the definition.
(thesaurus)
'step missing a' is the wordplay.
'step' becomes 'stair' ('stair' can be a synonym of 'step').
'missing' is a deletion indicator.
'stair' with 'a' taken out is 'STIR'.
(Other definitions for stir that I've seen before include "Mix the ingredients" , "Slight movement" , "Commotion; agitate" , "Whisk, blend" , "Arouse; commotion" .)