Check up on marine working to penetrate Green Mountain State (7)
I believe the answer is:
vermont
'green mountain state' is the definition.
(synonyms)
'check up on marine working to penetrate' is the wordplay.
'check up' becomes 'vet' (I've seen this before. I am not sure about the 'up' bit.).
'on' indicates putting letters inside (as in clothing 'on' a person).
'marine' becomes 'rm'.
'working' becomes 'on' (on can mean working or activated).
'to penetrate' is an insertion indicator.
'vet' going around 'rm' is 'vermt'.
'vermt' going around 'on' is 'VERMONT'.
Can you help me to learn more?
(Other definitions for vermont that I've seen before include "New England state, capital Montpelier" , "The Green Mountain state" , "US state, named for the Green Mountains" .)