Thickset like a St Bernard rescue dog? (6-7)

I believe the answer is:

barrel-chested

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

'thickset' is the definition.
(both can mean stocky)

'a st bernard rescue dog?' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
'st' is within the answer.
This explanation may well be incorrect...

'like' is the link.

Can you help me to learn more?

I've seen this clue in The Telegraph.

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