Extremely selfish, muscular Atlas did (8)
I believe the answer is:
shrugged
'atlas did' is the definition.
(reference to the Ayn Rand novel Atlas Shrugged)
'extremely selfish muscular' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'muscular' becomes 'rugged' (rugged can mean tough).
'selfish' with its centre taken out is 'sh'.
'sh'+'rugged'='SHRUGGED'
(Other definitions for shrugged that I've seen before include "Raised and lowered shoulders to show indifference" , "Raised one's shoulders to show indifference or resignation" , "Raised and dropped shoulders to indicate indifference" , "Raised one's shoulders to show indifference or ignorance" , "Hunched the shoulders, showing indifference say" .)