Officers, that is, regarding underwear (9)
I believe the answer is:
brassiere
'underwear' is the definition.
'officers that is regarding' is the wordplay.
'officers' becomes 'brass' (I've seen this in another clue).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'regarding' becomes 're' ('re' means regarding or about).
'brass'+'ie'+'re'='BRASSIERE'
(Other definitions for brassiere that I've seen before include "support" , "garment" , "Female undergarment supporter" .)