Started making a subterranean passage (5,3)
I believe the answer is:
under way
'started' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs can occasionally define each other.
'a subterranean passage' is the wordplay.
'a subterranean' becomes 'under' (I can't explain this - if you can you should give a lot more credence to this answer).
'passage' becomes 'way' (I've seen this in another clue).
'under'+'way'='UNDER WAY'
'making' is the link.
Can you help me to learn more?
(Other definitions for under way that I've seen before include "Might sewer network be" , "In motion (nautical)" , "In progress" , "Going" , "coming on" .)