Reckless fellow, mostly "cheeky”, not once losing heart (9)
I believe the answer is:
'reckless fellow' is the definition.
(buccaneer can mean a reckless person)
'mostly cheeky not once losing heart' is the wordplay.
'mostly' means to remove the last letter.
'cheeky' becomes 'buccal' (buccal means relating to the cheeks).
'not once' becomes 'never' (I've seen this before).
'losing heart' says to hollow out the word (remove centre letters).
'never' with its middle taken out is 'neer'.
'buccal' with its final letter removed is 'bucca'.
(Other definitions for buccaneer that I've seen before include "'Sea Rover'" , "Sea wolf" , "Sea robber" , "Unscrupulous opportunist" , "Corsair" .)