Underwear that is going to stay on (6)
I believe the answer is:
linger
'stay on' is the definition.
(thesaurus)
'underwear that is going' is the wordplay.
'underwear' becomes 'lingerie' (type of underwear).
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'going' suggests deleting specific letters.
'lingerie' with 'ie' taken away is 'LINGER'.
'to' acts as a link.
(Other definitions for linger that I've seen before include "Remain behind" , "Hang about, don't be in a hurry to go" , "Be slow to leave" , "'Loiter, delay leaving (6)'" , "Tarry, hang around" .)