Four obliged to join a decentralised rally (8)
I believe the answer is:
boundary
'four' is the definition.
(cricket term)
'obliged to join a decentralised rally' is the wordplay.
'obliged' becomes 'bound' ('bind' can be a synonym of 'oblige').
'to join' means one lot of letters go next to another.
'decentralised' means to remove the middle letters (the centre of the word is removed).
'rally' with its middle removed is 'ry'.
'bound'+'a'+'ry'='BOUNDARY'
(Other definitions for boundary that I've seen before include "Frontier" , "4 possibly" , "Four, perhaps" , "Line determining the limits of an area" , "Six, perhaps" .)