Busily working in the navy, maybe (2,6,7)
I believe the answer is:
on active service
'busily' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'working in the navy maybe' is the wordplay.
I cannot really see how this works, but
'working' could be 'active' (similar in meaning) and 'active' is found within the answer.
'navy' could be 'n' (common abbreviation eg in RN Royal Navy) and 'n' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for on active service that I've seen is " engaged in military operation".)