Ships in an old London river (5)
I believe the answer is:
fleet
'ships' is the definition.
(eg a naval fleet)
'in an old london river' is the wordplay.
'in' indicates putting letters inside.
'an old london' becomes 'ft' (I can't explain this - if you can you should believe this answer much more).
'river' becomes 'lee' (I've seen this in other clues).
'ft' going around 'lee' is 'FLEET'.
Can you help me to learn more?
(Other definitions for fleet that I've seen before include "Group of planes, ships or cars" , "vehicles owned by a company" , "Squadron" , "task force" , "Body of ships" .)