Postscript to short poem written by captain is mostly outstanding (8)
I believe the answer is:
epilogue
'postscript' is the definition.
(thesaurus)
'captain is mostly outstanding' is the wordplay.
'captain' becomes 'epilog' (I can't justify this - if you can you should give a lot more credence to this answer).
'is mostly' means to remove the first letter.
'outstanding' becomes 'due' (I've seen this before).
'due' with its first letter taken away is 'ue'.
'epilog'+'ue'='EPILOGUE'
'to short poem written by' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for epilogue that I've seen before include "Closing section" , "Postscript to a literary work" , "Late religious message" , "Postscript to play, book etc" , "Short passage added at the end of a literary work" .)