Fashionable black suit's a great deal (2,6)
I believe the answer is:
in spades
'a great deal' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'fashionable black suit's' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'black suit' becomes 'spades' (I've seen this before).
'in'+'spades'='IN SPADES'
Can you help me to learn more?
(Other definitions for in spades that I've seen before include "To a very great degree" , "Extremely" , "how a contract may be agreed" , "Very much so" , "Unquestionably - how a bridge hand might be played?" .)