Valuable - but too nice (8)
I believe the answer is:
precious
This clue could be a double definition.
'valuable' is the first definition.
(similar in meaning)
'too nice' is the second definition.
'precious' can be an answer for 'nice' (thesaurus). I am not certain of the 'too' bit.
'but' is the link.
Can you help me to learn more?
(Other definitions for precious that I've seen before include "Very dear" , "Having great value" , "Much loved" , "Valuable or dearly beloved" , "Of high worth or cost" .)