Medium lycra pants and cape for one to wear (7)
I believe the answer is:
acrylic
'medium' is the definition.
'lycra pants and cape for one to wear' is the wordplay.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'and' means one lot of letters go next to another.
'cape' becomes 'c' (abbreviation used on maps).
'for one' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to wear' means one lot of letters goes inside another.
'lycra' anagrammed gives 'acryl'.
'acryl'+'c'='acrylc'
'acrylc' going around 'i' is 'ACRYLIC'.
(Other definitions for acrylic that I've seen before include "Type of hard paint" , "Textile fibre - icy Carl (anag)" , "The synthetic fibre is icy, Carl" , "Synthetic resin or fibre used for textiles" , "Synthetic fibre or paint" .)