Worker in a tube extension? (3)
I believe the answer is:
bee
'worker' is the definition.
(a worker bee)
'in a tube extension?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'BEE' is hidden in the letters of 'tube extension'.
(Other definitions for bee that I've seen before include "Honey generator" , "meeting" , "Winged insect" , "Hive worker" , "Flying insect (often busy)" .)