Underwear is itching, gents admitted (4,5)
I believe the answer is:
long johns
'underwear' is the definition.
(I've seen this before)
'is itching gents admitted' is the wordplay.
'is itching' becomes 'longs' (as in longing or itching for something).
'gents' becomes 'john' (john can informally mean a toilet).
'admitted' indicates putting letters inside (inserted letters are admitted in).
'longs' going around 'john' is 'LONG JOHNS'.
(Other definitions for long johns that I've seen before include "Thermal underpants" , "Old habits" , "warm clothing" , "Underpants with legs" , "So-called pirate's" .)