Heard aboard loose ships making slogan for mime (3-4)
I believe the answer is:
lip-sync
'mime' is the definition.
(to lip-sync is to mime along to a song)
I can't explain the rest of the clue.
(Other definitions for lip-sync that I've seen before include "Linking of speaker's mouth movements and voice in filmed recordings" , "Align mouth movements with recorded speech" , "Audio and video alignment of speech" .)