Stop in case of efficiency and fairness (6)
I believe the answer is:
equity
'fairness' is the definition.
('equity' can be a synonym of 'fairness')
'stop in case of efficiency' is the wordplay.
'stop' becomes 'quit' ('quit' can be a synonym of 'stop'**).
'in' is an insertion indicator.
'case of' suggests removing the centre (outsides of).
'efficiency' with its centre removed is 'ey'.
'quit' placed into 'ey' is 'EQUITY'.
'and' is the link.
(Other definitions for equity that I've seen before include "actors' group" , "Even-handedness" , "Fairness, impartial treatment" , "Fairness; shareholding" , "Fair play - actors' union" .)