Tailor’s small boat (6)
I believe the answer is:
cutter
'boat' is the definition.
(cutter is a kind of sailing vessel)
'tailor's small' is the wordplay.
'tailor' becomes 'cut' (synonyms).
'small' becomes 'ter' (I am not sure about this - if you are sure you should believe this answer much more).
'cut'+'ter'='CUTTER'
Can you help me to learn more?
(Other definitions for cutter that I've seen before include "One working with Ruby possibly" , "Small fast sailing ship - sounds sharp" , "Boat; tailor" , "A scissors, say, or a small ship" , "Fast patrol boat" .)