Old alcoholic finally sober in support group (7)
I believe the answer is:
octette
'group' is the definition.
(octette is a kind of group)
'old alcoholic finally sober in support' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'finally' suggests the final letters.
'sober' becomes 'tt' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'support' becomes 'tee' (I've seen this before**).
The final letter of 'alcoholic' is 'c'.
'tt' put into 'tee' is 'tette'.
'o'+'c'+'tette'='OCTETTE'
(Other definitions for octette that I've seen before include "Noted work" , "Group of eight" , "Eight musicians" .)