Sitting down at Strike’s invitation, Bristow looked round the tatty office in what Strike was afraid was disappointment. The prospective client seemed nervous in the guilty way that Strike had come to associate with suspicious husbands, yet a faint air of authority clung to him, conveyed mainly by the obvious expense of his suit. Strike wondered how Bristow had found him. It was hard to get word-of-mouth business when your only client (as she regularly sobbed down the telephone) had no friends. (The Cuckoo's Calling, by Robert Galbraith)
The preposition, in, and the fused relative, what, in the highlighted sentence makes me puzzled. It seems to me that ‘in what [Strike was afraid] was disappointment’ means ‘in the way whose looking around his office room disappointed, to Strike, and of that Strike was afraid. What does it mean?