A width has sadly become flooded (5)
I believe the answer is:
awash
'flooded' is the definition.
(synonyms)
'a width has sadly' is the wordplay.
'width' becomes 'w' (abbreviation in geometry).
'sadly' is an anagram indicator.
'has' is an anagram of 'ash'.
'a'+'w'+'ash'='AWASH'
'become' is the link.
(Other definitions for awash that I've seen before include "Flooded with water" , "Level with the water surface" , "Flood" , "Overflowing" , "Flooded; about to sink" .)