Frail mare disturbed by warning device (4,5)
I believe the answer is:
fire alarm
'warning device' is the definition.
(fire alarm is a kind of warning device)
'frail mare disturbed' is the wordplay.
'disturbed' indicates anagramming the letters (the order of letters is disturbed).
'frail'+'mare'='frailmare'
'frailmare' is an anagram of 'FIRE-ALARM'.
'by' is the link.
(Other definitions for fire alarm that I've seen before include "Warner" , "Warning to evacuate" , "Warning device" , "Safety equipment" , "Safety device" .)