Tough as Cassidy? (5)
I believe the answer is:
butch
'tough' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for butch that I've seen before include "Macho" , "Aggressively masculine" , "Tough" , "Markedly masculine" , "Acting as a male (colloq.)" .)