Frank is about one's height (6)
I believe the answer is:
honest
'frank' is the definition.
(similar in meaning)
'about one's height' is the wordplay.
'about' is an insertion indicator.
'height' becomes 'ht' (I've seen this in other clues).
'ones' inserted within 'ht' is 'HONEST'.
'is' acts as a link.
(Other definitions for honest that I've seen before include "Genuine - I promise you!" , "Honourable, true" , "Fair and free of deceit" , "Direct" , "Reliable - the son (anag)" .)