Fleet when moored? (4)
I believe the answer is:
fast
I believe this clue is a double definition.
'fleet' is the first definition.
(similar in meaning)
'moored?' is the second definition.
(thesaurus)
'when' acts as a link.
(Other definitions for fast that I've seen before include "Rapid; abstain from food" , "Relating to the mind" , "Firm < moving rapidly" , "Moored - fleet" , "Eat nothing" .)