Pirate's leader aware of refusal to turn back boat (7)
I believe the answer is:
pontoon
'boat' is the definition.
(I know that pontoon is a type of barge)
'pirate's leader aware of refusal to turn back' is the wordplay.
'leader' indicates taking the first letters.
'aware of' becomes 'onto' (if you're onto someone you know what they're up to).
'refusal' becomes 'no'.
'to turn back' says the letters should be written in reverse (I've seen 'turned back' mean this).
The first letter of 'pirates' is 'p'.
'no' reversed gives 'on'.
'p'+'onto'+'on'='PONTOON'
(Other definitions for pontoon that I've seen before include "Vingt-et-un" , "Card game; floating support" , "Gambling game" , "Blackjack" , "Floating bridge support" .)