In truth, male is hairless (6)
I believe the answer is:
'hairless' is the definition.
(similar in meaning)
'in truth male' is the wordplay.
'in' means one lot of letters goes inside another.
'truth' becomes 'sooth' (synonyms).
'male' becomes 'm' (common abbreviation).
'sooth' placed around 'm' is 'SMOOTH'.
'is' acts as a link.
(Other definitions for smooth that I've seen before include "Unruffled" , "Level, not lumpy" , "Lacking roughness" , "Regular - without problems" , "Level; suave" .)