Rodent brother heard under the counter (8)
I believe the answer is:
chipmunk
'rodent' is the definition.
(chipmunk is a kind of rodent)
'brother heard under the counter' is the wordplay.
'brother' becomes 'monk' (I've seen this before).
'heard' indicates a 'sounds like' (homophone) clue.
'under' means one lot of letters go next to another.
'the counter' becomes 'chip' (chip is a kind of counter).
'monk' sounds like 'munk'.
'munk' put after 'chip' is 'CHIPMUNK'.
(Other definitions for chipmunk that I've seen before include "Burrowing squirrel with cheek pouches" , "American creature" , "Squirrel - much pink (anag)" , "Animal" , "US ground squirrel" .)