Try on extremely stiff new pants (1-6)
I believe the answer is:
y-fronts
'pants' is the definition.
(I've seen this before)
'try on extremely stiff new' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'new' is an anagram indicator.
'stiff' with its centre taken out is 'sf'.
'try'+'on'+'sf'='tryonsf'
'tryonsf' anagrammed gives 'Y-FRONTS'.
(Other definitions for y-fronts that I've seen before include "Type of underpants" , "Men's pants" , "Item of male clothing" , "Pants for men" , "Male undergarment" .)