Thoughts about Sadie (5)
I believe the answer is:
ideas
'thoughts' is the definition.
(I know this)
'about sadie' is the wordplay.
'about' is an anagram indicator.
'sadie' anagrammed gives 'IDEAS'.
(Other definitions for ideas that I've seen before include "Notions, thoughts" , "Notions, concepts" , "Thoughts, maybe creative" , "thinking" , "Brainwaves" .)