Thin and puny (5)
I believe the answer is:
weedy
'thin and puny' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for weedy that I've seen before include "Underdeveloped" , "Skinny" , "Scraggy" , "Thin, puny (colloq.)" , "wimpish" .)