Frightfully honest — extremely lazy in secret (2,3,3)
I believe the answer is:
on the sly
Here is my best explanation:
'in secret' is the definition.
('on the sly' can be a synonym of 'in secret')
'frightfully honest extremely lazy' is the wordplay.
'frightfully' is an anagram indicator (I've seen this in other clues).
'extremely' says to hollow out the word (remove centre letters).
'lazy' with its centre taken out is 'ly'.
'honest' with letters rearranged gives 'onthes'.
(Other definitions for on the sly that I've seen before include "Honestly (anag) - not honestly", "Surreptitiously", "Honestly (anag) - dishonestly", "Secretly or furtively", "Under the table", "With craft", "Furtively", "Behind someone's back", "in private" and "In secretive fashion".)