Mop was confused with broom, initially (4)
I believe the answer is:
swab
'mop' is the definition.
(synonyms)
'was confused with broom initially' is the wordplay.
'confused with' indicates anagramming the letters.
'initially' indicates taking the first letters.
The initial letter of 'broom' is 'b'.
'was' with its letters mixed up with 'b' gives 'SWAB'.
(Other definitions for swab that I've seen before include "Small piece of cotton used to cleanse a wound" , "Absorbent pad for cleansing or drying" , "Pad used in surgery" , "Surgical cleaner" , "Mop up - uncouth fellow" .)