Badger joined other burrowers on opening of tunnel (6)
I believe the answer is:
molest
'badger' is the definition.
(I know that badger is a kind of molest)
'on opening of tunnel' is the wordplay.
'on' means one lot of letters go next to another.
'opening of' becomes 'st' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tunnel' becomes 'mole' (I've seen this in another clue).
'st' put after 'mole' is 'MOLEST'.
'joined other burrowers' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for molest that I've seen before include "Ghost" , "Accost in motels" , "Harass" , "Pester unpleasantly" , "Pester; assault" .)