Stiff mud is not even liquefied with tar (15)
I believe the answer is:
undemonstrative
'stiff' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'mud is not even liquefied with tar' is the wordplay.
'liquefied with' indicates an anagram (a fluid or liquid form of the letters).
'mud'+'is'+'not'+'even'='mudisnoteven'
'mudisnoteven' mixed up with 'tar' makes 'UNDEMONSTRATIVE'.
Can you help me to learn more?
(Another definition for undemonstrative that I've seen is " Cold".)