Ship about beginning to raise flag (8)

I believe the answer is:

streamer

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'flag' is the definition.
(I know that streamer is a type of flag)

'ship about beginning to raise' is the wordplay.
'ship' becomes 'seamer' (I can't explain this - if you can you should believe this answer much more).
'about' indicates putting letters inside.
'beginning' indicates taking the first letters.
The initial letters of 'to raise' is 'tr'.
'seamer' placed around 'tr' is 'STREAMER'.

Can you help me to learn more?

(Other definitions for streamer that I've seen before include "Long flag or decoration" , "Banner headline; long strip" , "awful cold?" , "Ribbon for throwing" , "Long strip used at parties" .)

I've seen this clue in The Telegraph.

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