Birds -- gosh! -- going over the Home Counties (5)
I believe the answer is:
geese
'birds' is the definition.
(goose is a kind of bird)
'gosh going over the home counties' is the wordplay.
'gosh' becomes 'gee' (I've seen this before**).
'going over' is a charade indicator (letters next to each other).
'the home counties' becomes 'se'.
'gee'+'se'='GEESE'
(Other definitions for geese that I've seen before include "Long-necked, migratory aquatic birds" , "Birds in a gaggle" , "Large water-birds with webbed feet and broad bills" , "Water birds; fools" , "Large birds of the duck family" .)