Answer: ATATIME

AnswerCrossword Clue
ATATIMEOne ... (singly).
ATATIMEOne .... (singly)
ATATIMEOne day ....
ATATIMECompletion of "one day"
ATATIMEOrderly phrase that can follow one, two or three
ATATIME"One Day ...," TV sitcom
ATATIMEOne thing ...
ATATIMEOne ... (singly)
ATATIMEOne ...
ATATIMESimultaneously
ATATIME"Stick to one thing ...."
ATATIMEOne ....
ATATIME"... like this . . . "
ATATIMEConcurrently
ATATIMETogether
ATATIMEPer instance
ATATIMEUnit by unit, in succession