Promising when back, ourselves, to call on (8)

I believe the answer is:

assuring

'promising' is the definition.
('assure' can be a synonym of 'promise')

'when back ourselves to call on' is the wordplay.
'when' becomes 'as'.
'back' says the letters should be written backwards.
'ourselves' becomes 'us'.
'to call' becomes 'ring' ('ring' can be a synonym of 'call'**).
'on' says to put letters next to each other.
'us' backwards is 'su'.
'as'+'su'+'ring'='ASSURING'

I've seen this clue in the Evening Standard.

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