Good horse getting agitated around quiet rodents (7)
I believe the answer is:
gophers
'rodents' is the definition.
(gopher is a kind of rodent)
'good horse getting agitated around quiet' is the wordplay.
'good' becomes 'g' (abbreviation).
'getting agitated' indicates anagramming the letters.
'around' indicates putting letters inside.
'quiet' becomes 'p' (music symbol for 'piano').
'horse' is an anagram of 'ohers'.
'ohers' going around 'p' is 'ophers'.
'g'+'ophers'='GOPHERS'
(Other definitions for gophers that I've seen before include "Creatures" , "waffles" , "Computer browsers" .)