A previous captain who was very good (4,6)

I believe the answer is:

past master

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'was very good' is the definition.
I can't judge whether this defines the answer.

'a previous captain' is the wordplay.
'a previous' becomes 'past' (I've seen this in another clue).
'captain' becomes 'master' (synonyms).
'past'+'master'='PAST-MASTER'

'who' acts as a link.

Can you help me to learn more?

(Other definitions for past master that I've seen before include "Person experienced in a particular field" , "Thomas Arnold was" , "seen at staff reunions?" , "Someone with a long and thorough experience in a given activity" , "wizard" .)

I've seen this clue in the Evening Standard.

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