Decoration sported by a boater? (7)
I believe the answer is:
hatband
'decoration' is the definition.
(hatband is a kind of decoration)
'sported by a boater?' is the wordplay.
'sported' becomes 'band' (I can't justify this - if you can you should give a lot more credence to this answer).
'by' says to put letters next to each other.
'a boater?' becomes 'hat' (boater is a kind of hat).
'band' after 'hat' is 'HATBAND'.
Can you help me to learn more?
(Other definitions for hatband that I've seen before include "Bowler may have one" , "Headgear decoration" , "Part of headgear" .)