Peg keeps casks back - one's used in laboratory

'one's used in laboratory' is the definition.
I can't tell whether this definition defines the answer.

'peg keeps casks back' is the wordplay.
'peg' becomes 'tee' (tee is a kind of peg).
'keeps' indicates putting letters inside.
'casks' becomes 'butts' (butt is a kind of cask).
'back' says the letters should be written backwards.
'butts' back-to-front is 'sttub'.
'tee' placed around 'sttub' is 'TEST-TUBE'.

Can you help me to learn more?