In hold-up, gunman with regularly veiled accent (5)
I believe the answer is:
twang
'accent' is the definition.
(I've seen this before)
'in hold-up gunman with regularly veiled' is the wordplay.
'in hold-up' shows that the letters should be reversed in order.
'regularly veiled' indicates alternate letters.
'gunman'+'with'='gunmanwith'
The alternating letters of 'gunmanwith' are 'gnawt'.
'gnawt' reversed gives 'TWANG'.
(Other definitions for twang that I've seen before include "way of speaking" , "ringing sound" , "Strongly nasal quality in speech" , "Plucking sound - nasal tone" , "accent's nasal sound" .)