Action triggers moves, and moves continue as long as they need to, based on what's happening in the fiction. In some ways, this is going to need to get eyeballed by the group. In the case of Defend, the move is triggered when you "stand in defense of a person, item, or location". Your hold is valid "so long as you stand in defense".
Personally, I'd look external to the situation - is it a crazy hectic battle or is is small enough to allow a breath or two of fresh air? (This is really just "begin and end with the fiction"). If that's the case, you may not be Defending anymore in either case. Defend relies on an external source of danger to defend against, right? If it's not there, the Defense is over. If there is Danger, ignoring it to cast a spell may preempt all that business and require that you Defy Danger before you cast.
Alternately, if the Cleric wants to aid the ally, he should spend that hold for "Open up the attacker to an ally giving that ally +1 forward against the attacker" on a case-by-case basis.
Generally, I'd say you're defending or you're not. Taking another action means you're doing that, instead. Again, the situation is going to dictate the terms of the move.