Fifty scouts disturbed by insects (7)
I believe the answer is:
locusts
'insects' is the definition.
(locust is a kind of insect)
'fifty scouts disturbed by' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'disturbed' is an anagram indicator (the order of letters is disturbed).
'by' is a charade indicator (letters next to each other).
'scouts' is an anagram of 'ocusts'.
'l'+'ocusts'='LOCUSTS'
(Other definitions for locusts that I've seen before include "Destructive grasshoppers" , "Grasshoppers of warm regions" , "Strippers" , "can be a plague" .)