Rodent let out, causing alarm (6)
I believe the answer is:
rattle
'alarm' is the definition.
(rattle can mean to alarm or unnerve)
'rodent let out' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'let' with letters rearranged gives 'tle'.
'rat'+'tle'='RATTLE'
'causing' acts as a link.
(Other definitions for rattle that I've seen before include "Child's toy; unsettle" , "making one upset" , "Baby's toy; sound of loose part" , "flap" , "Unnerve - baby's plaything" .)