Not lying about one's height (6)
I believe the answer is:
honest
'not lying' is the definition.
(I've seen this before)
'about one's height' is the wordplay.
'about' means one lot of letters goes inside another.
'height' becomes 'ht' (abbreviation).
'ones' going into 'ht' is 'HONEST'.
(Other definitions for honest that I've seen before include "Honorable and sincere" , "Fair and square" , "Genuine; fair" , "Trustworthy or candid" , "really!" .)