Friend supporting infant? Absolutely! (7)
I believe the answer is:
totally
'absolutely' is the definition.
('totally' can be a synonym of 'absolutely')
'friend supporting infant?' is the wordplay.
'friend' becomes 'ally' (I've seen this before).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'infant?' becomes 'tot' (I've seen this before).
'ally' after 'tot' is 'TOTALLY'.
(Other definitions for totally that I've seen before include "Wholly" , "perfectly" , "Altogether" , "Allowing no exception" , "without reserve" .)