Clothing for panto with navy blue material (4)
I believe the answer is:
porn
'blue material' is the definition.
(I've seen this before)
'clothing for panto with navy' is the wordplay.
'clothing for' means to remove the middle letters (only the letters which clothe the word).
'with' means one lot of letters go next to another.
'navy' becomes 'RN' (abbreviation for Royal Navy).
'panto' with its middle removed is 'po'.
'po'+'rn'='PORN'
(Other definitions for porn that I've seen before include "Lewd material (abbr)" , "Obscene material" , "blue material" .)