Top-grade partners cutting post one intended to fill on board (8)
I believe the answer is:
mainsail
'one intended to fill on board' is the definition.
(a mainsail fills out with wind)
'top-grade partners cutting post' is the wordplay.
'top grade' becomes 'AI' (resembles 'A1', old-fashioned term for excellent).
'partners' becomes 'ns' (North and South are partners in the game bridge).
'cutting' is an insertion indicator (some letters cut their way into a word).
'post' becomes 'mail' (synonyms).
'ai'+'ns'='ains'
'ains' going into 'mail' is 'MAINSAIL'.
(Other definitions for mainsail that I've seen before include "Principal canvas sheet on ship" , "Ship's largest canvas" , "Lowest canvas on the principle mast" , "Ian's mail (anagram)" , "key equipment on board" .)