One talks about triceps primarily being shredded (2,7)
I believe the answer is:
in tatters
'being shredded' is the definition.
(I've seen this in another clue)
'one talks about triceps primarily' is the wordplay.
'one' becomes 'I' (Roman numeral).
'talks' becomes 'natters' (I can't explain this - if you can you should believe this answer much more).
'about' means one lot of letters goes inside another.
'primarily' says to take the initial letters.
The initial letter of 'triceps' is 't'.
'natters' placed around 't' is 'ntatters'.
'i'+'ntatters'='IN TATTERS'
Can you help me to learn more?
(Other definitions for in tatters that I've seen before include "Destroyed, torn" , "Torn to pieces" , "Urgently needing sewer" , "Treat isn't (anag) - ruined" , "Ragged" .)