Group that is to make an attack (6)
I believe the answer is:
sortie
'an attack' is the definition.
(I've seen this before)
'group that is' is the wordplay.
'group' becomes 'sort'.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'sort'+'ie'='SORTIE'
'to make' acts as a link.
(Other definitions for sortie that I've seen before include "Sudden attack by besieged troops" , "Military raid" , "Short trip; attack" , "Raid - short trip" , "Attack launched from place of siege" .)