Notice, when the train moves out, looking very happy (7)

I believe the answer is:

radiant

I'm a little stuck... Click here to teach me more about this clue!

'happy' is the definition.
(thesaurus)

'notice when the train moves out' is the wordplay.
'notice when' becomes 'ad' (I am not sure about the 'when' bit.**).
'moves' indicates anagramming the letters.
'out' is an insertion indicator.
'train' anagrammed gives 'riant'.
'ad' placed within 'riant' is 'RADIANT'.

'looking very' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for radiant that I've seen before include "Shining; joyful" , "with good humour" , "Straight line" , "Splendid; heater type" , "Effulgent" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.