Avoided being overheard in Tube (4)
I believe the answer is:
duct
'tube' is the definition.
(I've seen this before)
'avoided being overheard' is the wordplay.
'avoided' becomes 'ducked' (ducking is a kind of avoiding).
'being overheard' indicates a 'sounds like' (homophone) clue.
'ducked' sounds like 'DUCT'.
'in' acts as a link.
(Other definitions for duct that I've seen before include "Air passage" , "Tube for conveing cable, liquid etc" , "Pipe, tube" , "Tube, passageway" , "Tube for cables, air, etc" .)