To be fair to Nick Clegg (and I may be the only person in the UK at the moment who is!), he needs to regain control and respect within the Lib Dems.
The overall consensus in the party is that Trident should be a no-go, and Danny Alexander appears to be fairly popular and apparently dislikes Trident.
Putting Danny Alexander in to review Trident is a win-win for Clegg.
Since the Lib Dems are unlikely ever to be able to block Trident again, it all makes little difference.