A small seabird behind boat (6)
I believe the answer is:
astern
'a small seabird behind boat' is the definition.
I can't judge whether this definition defines the answer.
'a small seabird behind' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'seabird' becomes 'tern' (tern is a kind of seabird).
'behind' says to put letters next to each other.
'a'+'s'+'tern'='ASTERN'
Can you help me to learn more?
(Other definitions for astern that I've seen before include "Towards rear of a vessel" , "Behind (naut.)" , "State of S central US" , "back for sailors" , "Toward the tail of a vessel" .)