Guard keeps goon mostly in cage (9)
I believe the answer is:
enclosure
'cage' is the definition.
(thesaurus)
'guard keeps goon mostly' is the wordplay.
'guard' becomes 'ensure' (I can't explain this - if you can you should give a lot more credence to this answer).
'keeps' indicates putting letters inside.
'goon' becomes 'clod' (synonyms).
'mostly' means to remove the last letter.
'clod' with its last letter taken away is 'clo'.
'ensure' placed around 'clo' is 'ENCLOSURE'.
'in' is the link.
Can you help me to learn more?
(Other definitions for enclosure that I've seen before include "Area fenced off - one's ulcer (anag)" , "Document included with a letter" , "One's contained" , "Sealed-off area" , "pound" .)