Surrounded by a mass, working girl begins (5)
I believe the answer is:
among
'surrounded by' is the definition.
(I've seen this before)
'a mass working girl begins' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'working' becomes 'on'.
'begins' suggests taking the first letters.
The initial letter of 'girl' is 'g'.
'a'+'m'+'on'+'g'='AMONG'
(Other definitions for among that I've seen before include "when in company" , "In the midst of (others)" , "in a group" , "Go man! (anag.)" , "No mag (anag.)" .)