In the middle of a second new group initially (5)
I believe the answer is:
among
'in the middle of' is the definition.
(I've seen this before)
'a second new group initially' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'initially' says to take the initial letters.
The initial letters of 'new group' is 'ng'.
'a'+'mo'+'ng'='AMONG'
(Other definitions for among that I've seen before include "In an assembly of" , "in a group" , "Included with - inside" , "In the midst of (others)" , "In the number of" .)