Rough diamonds supporting hooligans' game, mostly (6)
I believe the answer is:
rugged
'rough' is the definition.
(similar in meaning)
'diamonds supporting hooligans' game mostly' is the wordplay.
'diamonds' becomes 'd' (abbreviation in bridge say).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'hooligans' game' becomes 'rugger' (short for rugby).
'mostly' means to remove the last letter.
'rugger' with its final letter taken off is 'rugge'.
'd' put after 'rugge' is 'RUGGED'.
(Other definitions for rugged that I've seen before include "fit for heavy duty" , "Craggy, rough but kindly" , "(Of a man) having attractively strong and rough features" , "Wearing a toupee" , "Might the hearthside be so" .)