A duck-filled bath turned over roughly (5)
I believe the answer is:
about
'roughly' is the definition.
('about' can be a synonym of 'roughly')
'a duck-filled bath turned over' is the wordplay.
'duck' becomes 'o'.
'filled' is an insertion indicator.
'bath' becomes 'tub' ('tub' can be a synonym of 'bath'**).
'turned over' is a reversal indicator.
'o' inserted into 'tub' is 'tuob'.
'tuob' reversed gives 'bout'.
'a'+'bout'='ABOUT'
(Other definitions for about that I've seen before include "nearby" , "Regarding" , "On the topic of" , "Almost; around" , "In conection with, all around" .)