Step across certain to be appreciated (9)
I believe the answer is:
Here is my best explanation:
'appreciated' is the definition.
('treasure' can be a synonym of 'appreciate')
'step across certain' is the subsidiary indication.
'step' becomes 'tread' (tread is a kind of step).
'across' is an insertion indicator.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'tread' going around 'sure' is 'treasured'.
'to be' is the link.
(Other definitions for treasured that I've seen before include "Cherished, held dear", "thought highly of", "Cherished, precious", "Loved", "Held dear", "Darling", "Valuable", "Much-valued", "Chersihed, held dear" and "Regarded as precious".)