Flatter bottom before sheer climbs (6,2)
I believe the answer is:
butter up
'flatter' is the definition.
(buttering up is a kind of flattering)
'bottom before sheer climbs' is the wordplay.
'bottom' becomes 'butt' ('butt' can be a synonym of 'bottom'**).
'before' is a charade indicator (letters next to each other).
'sheer' becomes 'pure' ('pure' can be a synonym of 'sheer').
'climbs' says the letters should be written backwards.
'pure' written backwards gives 'erup'.
'butt'+'erup'='BUTTER UP'
(Other definitions for butter up that I've seen before include "Flatter using common dairy product" , "Flatter using dairy product? (6, 2)" , "like 17 [SWEET TALK]" , "Work on" , "Flatter for a purpose" .)