Keen for us to enter Merchant Navy (5)
I believe the answer is:
mourn
'keen' is the definition.
(I've seen this in another clue)
'for us to enter merchant navy' is the wordplay.
'for us' becomes 'our' ('our' means belonging to us).
'to enter' is an insertion indicator.
'merchant navy' becomes 'mn' (this might be a standard abbreviation which I don't know about).
'our' inserted within 'mn' is 'MOURN'.
Can you help me to learn more?
(Other definitions for mourn that I've seen before include "Grieve for a death" , "Express sadness" , "Regret (a loss)" , "'Miss'" , "Lament loss" .)