Disagreement if red fence collapsed (10)
I believe the answer is:
difference
'disagreement' is the definition.
(difference is a kind of disagreement)
'if red fence collapsed' is the wordplay.
'collapsed' indicates an anagram.
'if'+'red'+'fence'='ifredfence'
'ifredfence' with letters rearranged gives 'DIFFERENCE'.
(Other definitions for difference that I've seen before include "row" , "Argument - dissimilarity" , "Diversity" , "Quarrel; remainder in subtraction" , "Discrimination" .)