Within reduced amount, partners are said to provide so-called key arrangement (6)
I believe the answer is:
qwerty
'so-called key arrangement' is the definition.
(qwerty keyboard)
'within reduced amount partners are said' is the wordplay.
'within' means one lot of letters goes inside another.
'reduced amount' becomes 'qty' (abbreviation for quantity).
'partners' becomes 'we' (West and East are partners in the game bridge).
'are said' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'we'+'r'='wer'
'qty' placed around 'wer' is 'QWERTY'.
'to provide' acts as a link.
(Other definitions for qwerty that I've seen before include "Describing a standard typing keyboard" , "Standard typewriter keyboard" , "Denoting a standard UK keyboard" .)