Almost darted back after this first group of four (6)
I believe the answer is:
tetrad
'group of four' is the definition.
(I've seen this in another clue)
'almost darted back after this first' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'back' shows that the letters should be reversed in order.
'after' says to put letters next to each other.
'this first' becomes 't' (1st letter of 'this').
'darted' with its last letter taken away is 'darte'.
'darte' in reverse letter order is 'etrad'.
'etrad' after 't' is 'TETRAD'.
(Other definitions for tetrad that I've seen before include "quartet" , "number of Gospel writers" , "Group or set of four" .)