Frightfully honest — extremely lazy in secret (2,3,3)
I believe the answer is:
on the sly
'in secret' is the definition.
('on the sly' can be a synonym of 'in secret')
'frightfully honest extremely lazy' is the wordplay.
'frightfully' indicates an anagram (I've seen this in other clues).
'extremely' suggests removing the centre.
'lazy' with its centre taken out is 'ly'.
'honest' anagrammed gives 'onthes'.
'onthes'+'ly'='ON THE SLY'
(Other definitions for on the sly that I've seen before include "Secretly or furtively" , "Under the table" , "Surreptitiously" , "Honestly (anag) - dishonestly" , "in private" .)