We stir dye when it's thin (5)
I believe the answer is:
weedy
'it's thin' is the definition.
(similar in meaning)
'we stir dye' is the wordplay.
'stir' indicates anagramming the letters.
'dye' is an anagram of 'edy'.
'we'+'edy'='WEEDY'
'when' acts as a link.
(Other definitions for weedy that I've seen before include "wimpish" , "quite unlike Charles Atlas" , "Skinny" , "Thin, puny (colloq.)" , "Underdeveloped" .)