Rodent is climbing instrument (5)
I believe the answer is:
sitar
'instrument' is the definition.
(sitar is a kind of instrument)
'rodent is climbing' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'rat'+'is'='ratis'
'ratis' back-to-front is 'SITAR'.
(Other definitions for sitar that I've seen before include "Indian stringed instrument" , "Long necked and often plucked" , "Plucked instrument" , "Stringed instrument of the subcontinent" , "Indian string instrument" .)