If turning black, don't tell the truth (3)
I believe the answer is:
fib
'don't tell the truth' is the definition.
I can't judge whether this definition defines the answer.
'if turning black' is the wordplay.
'turning' says the letters should be written backwards.
'black' becomes 'b' (abbreviation as in pencil classifications).
'if' back-to-front is 'fi'.
'fi'+'b'='FIB'
Can you help me to learn more?
(Other definitions for fib that I've seen before include "Story" , "Petty falsehood" , "Little white lie" , "One must be 20 [UNTRUE]" , "minor piece of fiction" .)