Ship moving slowly out of lake (4)
I believe the answer is:
argo
'ship' is the definition.
(ship in Greek myth)
'moving slowly out of lake' is the wordplay.
'moving slowly' becomes 'largo' (I've seen this before).
'out of' suggests deleting specific letters.
'lake' becomes 'L' (geographical abbreviation).
'largo' with 'l' taken out is 'ARGO'.
(Other definitions for argo that I've seen before include "Agro about the constellation" , "Ancient ship" , "Fabulous ship" , "Jason's craft" , "Ship used by Jason in his search for the Golden Fleece" .)