Little donkey, extremely scrawny (5)
I believe the answer is:
weedy
'scrawny' is the definition.
(I know that scrawny can be written as weedy)
'little donkey extremely' is the wordplay.
'little' becomes 'wee' (similar in meaning).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'donkey' with its middle taken out is 'dy'.
'wee'+'dy'='WEEDY'
(Other definitions for weedy that I've seen before include "like an untended flower bed?" , "quite unlike Charles Atlas" , "Scraggy" , "wimpish" , "Skinny" .)