It's hard not to be tender (5)
I believe the answer is:
tough
I believe this is a double definition.
'it's hard' is the first definition.
(synonyms)
'not to be tender' is the second definition.
('tough' is the opposite to 'tender')
(Other definitions for tough that I've seen before include "Taxing" , "Able to withstand wear and tear" , "Strict, uncompromising" , "Strong - ruffian" , "Made to withstand abuse" .)