North America circumnavigated by horse in the end (7)
I believe the answer is:
finally
'in the end' is the definition.
(synonyms)
'north america circumnavigated by horse' is the wordplay.
'north america' becomes 'na'.
'circumnavigated by' is an insertion indicator (circumnavigate can mean to travel around).
'horse' becomes 'filly' (I have seen 'Female horse ' mean 'filly' so perhaps 'horse' could also mean 'filly').
'na' inserted inside 'filly' is 'FINALLY'.
(Other definitions for finally that I've seen before include "At last" , "for ever" , "when all is said and done" .)