Need bit of joist in support for flag (9)
I believe the answer is:
blackjack
'flag' is the definition.
(I know that blackjack is a type of flag)
'need bit of joist in support' is the wordplay.
'need' becomes 'lack' (to lack something is to be in need of it).
'bit of' says to take the initial letters.
'in' means one lot of letters goes inside another.
'support' becomes 'back' (to back is to support or endorse).
The initial letter of 'joist' is 'j'.
'lack'+'j'='lackj'
'lackj' put within 'back' is 'BLACKJACK'.
'for' is the link.
(Other definitions for blackjack that I've seen before include "A card game in which the object is to beat the bank" , "Vingt-et-un - pirates' ensign" , "2 8 [CASINO GAME]" , "Gambling card game" , "Casino card game" .)