Underwear, without padding, so plain (5)
I believe the answer is:
basic
'plain' is the definition.
(I've seen this before)
'underwear without padding so' is the wordplay.
'underwear' becomes 'bra' (bra is a type of undergarment).
'without padding' means to remove the middle letters (without the middle letters which pad out the word).
'so' becomes 'sic' ('so' in Latin).
'bra' with its centre taken out is 'ba'.
'ba'+'sic'='BASIC'
(Other definitions for basic that I've seen before include "Simple - computer language" , "Spartan" , "'Primary, fundamental (5)'" , "No-frills" , "vital" .)