Bases - about fifty - for the ships (5)
I believe the answer is:
fleet
'ships' is the definition.
(eg a naval fleet)
'bases about fifty' is the wordplay.
'bases' becomes 'feet' ('foot' can be a synonym of 'base').
'about' is an insertion indicator.
'fifty' becomes 'l' (Roman numeral).
'feet' enclosing 'l' is 'FLEET'.
'for the' is the link.
(Other definitions for fleet that I've seen before include "Group of ships, aircraft etc" , "Navy - quick" , "Swift; group of ships" , "Body of ships" , "Fast and nimble" .)