Outgoing submarine docks in river (7)
I believe the answer is:
expense
'outgoing submarine docks in river' is the definition.
I can't judge whether this definition defines the answer.
'submarine docks in river' is the wordplay.
'submarine docks' becomes 'pens' (pen can mean a submarine base).
'in' indicates putting letters inside.
'river' becomes 'exe' (River Exe).
'pens' put within 'exe' is 'EXPENSE'.
Can you help me to learn more?
(Other definitions for expense that I've seen before include "Cost - price" , "Charge" , "What something costs" , "damage" , "Money spent in the course of work" .)