Pirate almost stealing gold coin (6)
I believe the answer is:
florin
'coin' is the definition.
(historical type of coin)
'pirate almost stealing gold' is the wordplay.
'pirate' becomes 'flint' (Captain Flint from Treasure Island).
'almost' means to remove the last letter (most of the word but not all of it).
'stealing' is an insertion indicator.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'flint' with its final letter taken away is 'flin'.
'flin' placed around 'or' is 'FLORIN'.
(Other definitions for florin that I've seen before include "Old 2-shilling piece" , "Former English coin" , "[SHILLINGS] ?" , "Former British silver coin" , "Old English coin worth 10p" .)