On screen, Eleanor leaves wild ponies semi-trained (7)

I believe the answer is:

broncos

'wild ponies semi-trained' is the definition.
(a bronco is a semi-trained horse)

'on screen eleanor leaves' is the wordplay.
'on screen eleanor' becomes 'Bron' (actress Eleanor Bron).
'leaves' becomes 'cos' (cos lettuce leaves).
'bron'+'cos'='BRONCOS'

(Another definition for broncos that I've seen is " Unreliable transport".)

I've seen this clue in The Guardian.

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