In that form, without reading the cited document, I cannot clearly understand if the citation refers to the parenthetical part, to the main one or both.
I'd thus rephrase the sentence, depending on the goal of the citation. If the citation refers to the main part, I'd write
Simplistically, the goal of the investor is to maximise return (XYZ, 2011), subject to risk constraints.
If the citation refers to both parts, I'd write
Simplistically, taking into account risk constraints, the goal of the investor is to maximise return (XYZ, 2011).
If the citation refers to the parenthetical part, I'd write
Simplistically, the goal of the investor is to maximise return (subject to risk constraints, see XYZ, 2011).
or
Simplistically, the goal of the investor is to maximise return, subject to risk constraints (XYZ, 2011).