It's of material interest to dressmakers (5)
I believe the answer is:
satin
'interest to dressmakers' is the definition.
I can't judge whether this definition defines the answer.
'it's of material' is the wordplay.
'it' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'of material' becomes 'tin' (I can't justify this - if you can you should give a lot more credence to this answer).
'sa'+'tin'='SATIN'
Can you help me to learn more?
(Other definitions for satin that I've seen before include "Fine material" , "Saint (anag.)" , "Stain on silky cloth" , "Stain on shiny fabric" , "Shiny solk" .)