Equivalent muscle in mammal moving in opposite direction (7)

I believe the answer is:

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'mammal moving in opposite direction' is the definition.
I can't tell whether this defines the answer.

'equivalent muscle in' is the wordplay.
'equivalent' becomes 'reed' (I am not sure about this - if you are sure you should believe this answer much more).
'muscle' becomes 'lat' (short for latissimus muscle).
'in' indicates putting letters inside.
'reed' enclosing 'lat' is 'RELATED'.

Can you help me to learn more?

(Other definitions for related that I've seen before include "within family" , "Treadle (anag) - akin" , "were sympathetic" , "Told (a story etc)" , "Kin; connected" .)

I've seen this clue in The Telegraph.

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