Rough day on a flat boat (5)
I believe the answer is:
draft
'rough' is the definition.
(synonyms)
'day on a flat boat' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'on' says to put letters next to each other.
'a flat boat' becomes 'raft' (I've seen this before).
'd'+'raft'='DRAFT'
(Other definitions for draft that I've seen before include "Preliminary drawing" , "form of conscription" , "book's outline" , "Written plan" , "Early version of text" .)