Mole's evacuated tunnel, running off initially underground (5)
I believe the answer is:
metro
'underground' is the definition.
('metro' can be a synonym of 'underground')
'mole's evacuated tunnel running off initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'moles evacuated tunnel running off' is 'METRO'.
(Other definitions for metro that I've seen before include "Parisian rail system" , "The Paris Underground transport system" , "Parisian underground system" , "Paris subway" , "what moves many Parisians" .)