Keen army medic on vessel (5)
I believe the answer is:
mourn
'keen army' is the definition.
'mourn' can be an answer for 'keen' (I've seen this before). I'm unsure of the 'army' bit.
'medic on vessel' is the wordplay.
'medic' becomes 'mo' (medical officer).
'on' means one lot of letters go next to another.
'vessel' becomes 'urn' (urn is a kind of vessel).
'mo'+'urn'='MOURN'
Can you help me to learn more?
(Other definitions for mourn that I've seen before include "Show sorrow for loss" , "Express sadness" , "Keen" , "Grieve for a death" , "'Miss'" .)