Mob bit of rough wearing G-string (6)
I believe the answer is:
throng
'mob' is the definition.
(I know that mob can be written as throng)
'bit of rough wearing g-string' is the wordplay.
'bit of' suggests taking the first letters.
'wearing' indicates putting letters inside.
'g-string' becomes 'thong' (type of underwear).
The initial letter of 'rough' is 'r'.
'r' put within 'thong' is 'THRONG'.
(Other definitions for throng that I've seen before include "Large gathering of people" , "A large crowd of people" , "Pack" , "Densely packed crowd" , "Dense crowd" .)