"That feels so-o-o good!" (3)
I believe the answer is:
aah
'that feels so-o-o good' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aah that I've seen before include "Expression of appreciation or amazement" , "Expression of surprise" , "regular partner of 3 [OOH]" .)