Is suspicious about the girl's investment schemes (9)

'investment schemes' is the definition.
The answer and definition can be both related to thinking as well as being plural nouns.
Perhaps they are linked in a way I don't understand?

'is suspicious about the girl's' is the wordplay.
'is suspicious' becomes 'trusts' (I am not sure about this - if you are sure you should believe this answer much more).
'about' says to put letters next to each other.
'the girl's' becomes 'Dis' (Di is short for Diana).
'trusts' after 'dis' is 'DISTRUSTS'.

Can you help me to learn more?