Rough sketch of simple boat on delta (5)
I believe the answer is:
draft
'rough sketch' is the definition.
(I've seen this before)
'simple boat on delta' is the wordplay.
'simple boat' becomes 'raft' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'raft' put after 'd' is 'DRAFT'.
'of' acts as a link.
(Other definitions for draft that I've seen before include "Early version of text" , "Current of air or preliminary version" , "Rough version" , "Preliminary version or sketch" , "form of conscription" .)