Marine is a force in extreme environment (9)
I believe the answer is:
seafaring
'marine' is the definition.
(thesaurus)
'a force in extreme environment' is the wordplay.
'force' becomes 'F' (symbol used in Physics e.g. f=ma).
'in' indicates putting letters inside.
'extreme environment' becomes 'searing' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'f'='af'
'af' going within 'searing' is 'SEAFARING'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for seafaring that I've seen before include "Just like Jack" , "Travelling the ocean" , "Travelling by ship" , "ocean travel" , "Sailor's way of life" .)