Climbing rodents fed with form of protein shake (7)
I believe the answer is:
stagger
'shake' is the definition.
(thesaurus)
'climbing rodents fed with form of protein' is the wordplay.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'rodents' becomes 'rats' (rat is a kind of rodent).
'fed with' indicates putting letters inside.
'form of protein' becomes 'egg' (eggs are a source of protein).
'rats' placed around 'egg' is 'reggats'.
'reggats' back-to-front is 'STAGGER'.
(Other definitions for stagger that I've seen before include "Walk as if drunk" , "move by someone under the table?" , "bowl over" , "Walk as if about to fall" , "Reel; arrange so as to be out of line" .)