Gear includes large tartan skirt (4)
I believe the answer is:
kilt
'tartan skirt' is the definition.
(kilt made of tartan material)
'gear includes large' is the wordplay.
'gear' becomes 'kit' (kit is a kind of gear).
'includes' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'kit' placed around 'l' is 'KILT'.
(Other definitions for kilt that I've seen before include "Checked menswear" , "Highlanders garment" , "Scotsman's attire" , "Mac's skirt" , "Highland garment" .)