An old truth about Mike's flat (6)
I believe the answer is:
smooth
'flat' is the definition.
(thesaurus)
'an old truth about mike's' is the wordplay.
'an old truth' becomes 'sooth' (synonyms. I am not sure about the 'old' bit.).
'about' means one lot of letters goes inside another.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'sooth' enclosing 'm' is 'SMOOTH'.
Can you help me to learn more?
(Other definitions for smooth that I've seen before include "Hairless" , "Easy" , "Regular - without problems" , "Even-surfaced" , "Free from roughness or irregularities" .)