About to enter the group (5)
I believe the answer is:
three
'group' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?
'about to enter the' is the wordplay.
'about to' becomes 're' (I've seen this in other clues).
'enter' means one lot of letters goes inside another.
're' going within 'the' is 'THREE'.
Can you help me to learn more?
(Other definitions for three that I've seen before include "Number of Furies or Norns" , "trilogy" , "One, one, one" , "Number of strikes before you're out" , "III" .)