Lessee isn't developing determination (10)
I believe the answer is:
steeliness
'determination' is the definition.
(I have seen 'cold determination' mean 'steeliness' so perhaps 'determination' could also mean 'steeliness')
'lessee isn't developing' is the wordplay.
'developing' indicates anagramming the letters (letters developed into a new form).
'lessee'+'isnt'='lesseeisnt'
'lesseeisnt' is an anagram of 'STEELINESS'.
(Other definitions for steeliness that I've seen before include "Hard nature" , "Trustworthy condition" , "cold determination" , "Firm determination" .)