Group's leader had dressed formally (6)
I believe the answer is:
gowned
'dressed formally' is the definition.
'gowned' can be an answer for 'dressed' (gowning is a kind of dressing). I am unsure of the 'formally' bit.
'group's leader had' is the wordplay.
'leader' says to take the initial letters.
'had' becomes 'owned' ('own' can be a synonym of 'have').
The first letter of 'group' is 'g'.
'g'+'owned'='GOWNED'
Can you help me to learn more?
(Another definition for gowned that I've seen is " Clerically robed".)