Knowing about shirt being really tough (6)
I believe the answer is:
steely
'really tough' is the definition.
(steely means very tough or ruthless)
'knowing about shirt' is the wordplay.
'knowing' becomes 'sly' (I've seen this in other clues).
'about' is an insertion indicator.
'shirt' becomes 'tee shirt' (tee shirt is a kind of shirt).
'sly' going around 'tee' is 'STEELY'.
'being' acts as a link.
(Other definitions for steely that I've seen before include "Gimlet-eyed" , "With (cold) resolve" , "Obdurate" , "Hard and metallic" , "Ruthless" .)