Step across certain to be appreciated (9)
I believe the answer is:
treasured
'appreciated' is the definition.
('treasure' can be a synonym of 'appreciate')
'step across certain' is the wordplay.
'step' becomes 'tread' ('tread' can be a synonym of 'step'**).
'across' indicates putting letters inside.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'tread' enclosing 'sure' is 'TREASURED'.
'to be' acts as a link.
(Other definitions for treasured that I've seen before include "Cherished, held dear" , "Valued" , "Cherished, precious" , "Loved" , "Prized, precious" .)