Owls flying towards county lose speed (4,4)
I believe the answer is:
slow down
'lose speed' is the definition.
(thesaurus)
'owls flying towards county' is the wordplay.
'flying' indicates anagramming the letters.
'towards' means one lot of letters go next to another (I've seen this in other clues).
'county' becomes 'down' (County Down in Northern Ireland).
'owls' is an anagram of 'slow'.
'slow'+'down'='SLOW-DOWN'
(Other definitions for slow down that I've seen before include "Lose velocity" , "Don't go so fast" , "Reduce speed" , "take it more easy" , "relax" .)