In the navy, he means a good deal to his friend (7)
I believe the answer is:
matelot
'in the navy he means a good' is the definition.
The definition suggests a singular noun which matches the answer.
'deal to his friend' is the wordplay.
I cannot really see how this works, but
'deal' could be 'lot' (synonyms) and 'lot' is found within the answer.
'friend' could be 'mate' (mate is a kind of friend) and 'mate' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for matelot that I've seen before include "One at sea" , "A lot met (anag) - sailor" , "One sailor" , "Sailor (slang from French)" , "Rating" .)