Be undersized and run down (8)
I believe the answer is:
belittle
'run down' is the definition.
(thesaurus)
'be undersized' is the wordplay.
'undersized' becomes 'little' (similar in meaning).
'be'+'little'='BELITTLE'
'and' acts as a link.
(Other definitions for belittle that I've seen before include "Disparage - minimise" , "Imply that person or thing is unimportant" , "Denigrate, disparage" , "Make to seem unimportant" , "abuse" .)