Magical cross-country joiner the French resettled in Ely (3,4)
I believe the answer is:
ley line
'magical cross-country joiner' is the definition.
(supposed line of energy across a landscape)
'the french resettled in ely' is the wordplay.
'the french' becomes 'le'.
'resettled' indicates an anagram (letters must be resettled in a new order).
'in'+'ely'='inely'
'inely' with letters rearranged gives 'yline'.
'le'+'yline'='LEY LINE'
(Other definitions for ley line that I've seen before include "Supposed ancient pathway linking prehistoric sites" , "Supposed link between ancient sites" , "spiritual connection?" .)