Came up about promotion that’s simply balanced (4-8)

I believe the answer is:

well-adjusted

'balanced' is the definition.

'came up about promotion that's simply' is the wordplay.
'came up' becomes 'welled' (welling is a kind of coming up).
'about' indicates putting letters inside.
'promotion' becomes 'ad' (short for advert).
'that's simply' becomes 'just' (synonyms).
'ad'+'just'='adjust'
'welled' enclosing 'adjust' is 'WELL-ADJUSTED'.

(Another definition for well-adjusted that I've seen is " Emotionally and mentally stable".)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app