Undergarment that is about below neck (9)
I believe the answer is:
brassiere
'undergarment' is the definition.
(brassiere is a kind of undergarment)
'that is about below neck' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'about' becomes 're' (regarding**).
'below' means one lot of letters go next to another.
'neck' becomes 'brass' (I've seen this before).
'ie'+'re'='iere'
'iere' put after 'brass' is 'BRASSIERE'.
(Other definitions for brassiere that I've seen before include "garment" , "Female undergarment supporter" , "support" .)