Bath covered with balls and glass cylinders (4,5)
I believe the answer is:
test tubes
'glass cylinders' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Maybe you can see an association between them that I can't see?
'bath covered with balls' is the wordplay.
'bath' becomes 'tub' (synonyms).
'covered with' is an insertion indicator.
'balls' becomes 'testes' ('testis' can be a synonym of 'ball').
'tub' inserted inside 'testes' is 'TEST-TUBES'.
'and' is the link.
Can you help me to learn more?
(Other definitions for test tubes that I've seen before include "they may contain chemicals" , "Lab vessels" , "Glass cylinders closed at one end, used in laboratory work" , "Laboratory equipment" , "Lab glassware" .)