Outfit including line in national dress (4)
I believe the answer is:
kilt
'national dress' is the definition.
'kilt' can be an answer for 'dress' (thesaurus). I am unsure of the 'national' bit.
'outfit including line' is the wordplay.
'outfit' becomes 'kit' (synonyms).
'including' indicates putting letters inside.
'line' becomes 'l' (used when specifying particular lines from a poem).
'kit' enclosing 'l' is 'KILT'.
'in' is the link.
Can you help me to learn more?
(Other definitions for kilt that I've seen before include "Bone; circle line" , "Tartan clothing item" , "Scottish costume" , "Checked menswear" , "Highlanders garment" .)