Good bid but not backsides (2,6)
I believe the answer is:
no trumps
'good bid' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'not backsides' is the wordplay.
'backsides' becomes 'rumps' ('rump' can be a synonym of 'backside').
'not'+'rumps'='NO-TRUMPS'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for no trumps that I've seen before include "Sort of contract" , "Call" .)