Tirade about a girl glowing with happiness (7)
I believe the answer is:
radiant
'glowing with happiness' is the definition.
(I've seen this before)
'tirade about a girl' is the wordplay.
'tirade' becomes 'rant' (both can mean an angry outburst).
'about' means one lot of letters goes inside another.
'a girl' becomes 'adi' ().
'rant' enclosing 'adi' is 'RADIANT'.
(Other definitions for radiant that I've seen before include "Very conspicuous" , "Brilliant" , "Resplendent" , "Incandescent" , "looking happy" .)