Flatter boys' leader, say, on winning (6,2)

I believe the answer is:

butter up

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'flatter' is the definition.
(buttering up is a kind of flattering)

'boys' leader say on winning' is the wordplay.
'leader' suggests taking the first letters.
'say on' becomes 'utter' (I've seen this before. I am not sure about the 'on' bit.**).
'winning' becomes 'up' (I've seen this in other clues).
The initial letter of 'boys' is 'b'.
'b'+'utter'+'up'='BUTTER UP'

Can you help me to learn more?

(Other definitions for butter up that I've seen before include "Flatter using common dairy product" , "Work on" , "Soft-soap" , "Flatter in order to get something" , "Flatter with milk product?" .)

I've seen this clue in The Times.

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