No.1 story about happiness (7)
I believe the answer is:
elation
'happiness' is the definition.
(I've seen this in another clue)
'no 1 story about' is the wordplay.
'no 1' becomes 'no i' (resembles No 1).
'story' becomes 'tale' (synonyms).
'about' is a reversal indicator.
'noi'+'tale'='noitale'
'noitale' reversed gives 'ELATION'.
(Other definitions for elation that I've seen before include "Feeling of intense delight" , "Ecstasy -- toenail (anag)" , "Exaltation" , "Feeling of joy and pride" , "Feeling of joy for it alone" .)