Rats on board tucked into cream for the most part (7)
I believe the answer is:
betrays
'rats on' is the definition.
(I've seen this in another clue)
'board tucked into cream for the most part' is the wordplay.
'board' becomes 'tray'.
'tucked into' is an insertion indicator.
'cream' becomes 'best' (I've seen this in another clue).
'for the most part' means to remove the last letter (most but not all of the letters).
'best' with its final letter taken off is 'bes'.
'tray' inserted inside 'bes' is 'BETRAYS'.
(Other definitions for betrays that I've seen before include "Shows unintentionally, say feelings" , "shops" , "Gives away" , "Sells" , "Double-crosses" .)