I'm to turn out, come along (7)

I believe the answer is:

improve

'come along' is the definition.
(thesaurus)

'i'm to turn out' is the wordplay.
'to turn out' becomes 'prove' ('prove' can be a synonym of 'turn out').
'im'+'prove'='IMPROVE'

(Other definitions for improve that I've seen before include "Become better" , "Ameliorate" , "Get or make better" , "Polish" , "Change for the better" .)

I've seen this clue in The Telegraph.

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