A designer's initial put on clothing label (7)
I believe the answer is:
address
'label' is the definition.
(addressing is a kind of labelling)
'a designer's initial put on clothing' is the wordplay.
'initial' indicates taking the first letters.
'put on' means one lot of letters go next to another.
'clothing' becomes 'dress' (dress is a kind of clothing**).
The initial letter of 'designers' is 'd'.
'a'+'d'+'dress'='ADDRESS'
(Other definitions for address that I've seen before include "Think about" , "Call" , "Form of identification for building" , "Treat" , "personal details" .)