The most common and most likely answer to this crossword clue is the 6 letter word STEELY
I've seen this crossword clue in Financial Times
S
T
E
E
L
Y
More clues leading to the result STEELY
- Cold-blooded fish occupying dirty home
- Blessed female, see, is very determined
- Cold and unyielding saint, fishy?
- . Fixed enclosure containing fish
- Tough swimmer in dirty hole
- Determined, picking up bargain at end of day
- Metallic fish in hovel
- Strong shelter to west protected by enclosure
- Hard sell that executives initially see
- Determined, crafty little supporter within
Buddy explains!

The straight part
We know that "Tough" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tough = steely.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does STEELY mean?
STEELY adjective- Having qualities resembling those of steel, especially hard and resolute.
- Made of steel.