Mizzen or topgallant, say (4)
I believe the answer is:
sail
I believe this clue is a double definition.
'mizzen' is the first definition.
(thesaurus)
'topgallant say' is the second definition.
(I know that topgallant is a type of sail)
(Other definitions for sail that I've seen before include "Glide along" , "Ship's canvas" , "Put to sea in yacht" , "Navigate" , "Be 4 [SEABORNE]" .)