Even though deeper than US state, it's a group of mountains (6)
I believe the answer is:
massif
'a group of mountains' is the definition.
(I've seen this in another clue)
'even though deeper than us state' is the wordplay.
I cannot really see how this works, but
'though' could be 'if' (I've seen this in other clues) and 'if' is found within the answer.
'state' could be 'mass' (abbreviation for Massachusetts) and 'mass' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'it's' is the link.
Can you help me to learn more?
(Other definitions for massif that I've seen before include "Compact grouping of mountains" , "Geological mountain formation" , "Mountainous block" , "Highland area?" , "mountain peak" .)