Secure both ends of excavation on slope (6)
I believe the answer is:
enlist
'secure' is the definition.
(I know that enlist is a type of secure)
'ends of excavation on slope' is the wordplay.
'ends of' suggests removing the centre.
'on slope' becomes 'list' (I can't justify this - if you can you should give a lot more credence to this answer).
'excavation' with its middle removed is 'en'.
'en'+'list'='ENLIST'
'both' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for enlist that I've seen before include "Enrol in armed forces" , "Listen (anag.)" , "Engage" , "Join the armed services" , "Join up in silent way" .)